Fix warnings/errors reported by static code analysers

[shmsr]

Tools used: go vet, staticcheck, golangci-lint

Fixed some basic, small errors such as:

1. Remove unnecessary use of fmt.Sprintf when arg is already a string
2. Check errors
3. Remove dead code blocks
4. Remove unreachable code
5. Don't put punctuation or new after error messages (in packages)

Sample output from golangci-lint, staticcheck:

• staticcheck:

wsdl/wsdl.go:59:25: the argument is already a string, there's no need to use fmt.Sprintf (S1025)
wsdl/wsdl.go:61:9: the argument is already a string, there's no need to use fmt.Sprintf (S1025)
wsdlgen/cli.go:44:10: error strings should not be capitalized (ST1005)
wsdlgen/cli.go:44:10: error strings should not end with punctuation or a newline (ST1005)
wsdlgen/config.go:42:20: func (*Config).publicName is unused (U1000)
xmltree/xmltree.go:272:12: error strings should not be capitalized (ST1005)
xmltree/xmltree_test.go:284:3: the surrounding loop is unconditionally terminated (SA4004)
xsd/parse.go:285:11: error strings should not be capitalized (ST1005)
xsd/parse.go:364:11: error strings should not end with punctuation or a newline (ST1005)
xsdgen/cli.go:104:10: error strings should not be capitalized (ST1005)
xsdgen/cli.go:104:10: error strings should not end with punctuation or a newline (ST1005)
xsdgen/config.go:49:20: func (*Config).errorf is unused (U1000)
xsdgen/config.go:438:16: error strings should not be capitalized (ST1005)
xsdgen/config.go:493:8: const soapenc is unused (U1000)
xsdgen/xsdgen.go:300:11: error strings should not end with punctuation or a newline (ST1005)
xsdgen/xsdgen.go:937:20: func (*Config).addTokenListMethods is unused (U1000)

• golangci-lint:

wsdl/wsdl.go:18:2: `mimeNS` is unused (deadcode)
	mimeNS    = "https://schemas.xmlsoap.org/wsdl/mime/"
	^
wsdl/wsdl.go:19:2: `soapencNS` is unused (deadcode)
	soapencNS = "https://schemas.xmlsoap.org/soap/encoding/"
	^
wsdl/wsdl.go:20:2: `soapenvNS` is unused (deadcode)
	soapenvNS = "https://schemas.xmlsoap.org/soap/envelope/"
	^
wsdl/wsdl.go:21:2: `xsiNS` is unused (deadcode)
	xsiNS     = "https://www.w3.org/2000/10/XMLSchema-instance"
	^
wsdl/wsdl.go:22:2: `xsdNS` is unused (deadcode)
	xsdNS     = "https://www.w3.org/2000/10/XMLSchema"
	^
xsd/xsd.go:29:2: `schemaInstanceNS` is unused (deadcode)
	schemaInstanceNS = "https://www.w3.org/2001/XMLSchema-instance"
	^
xmltree/marshal.go:90:12: Error return value of `e.w.Write` is not checked (errcheck)
		e.w.Write([]byte("<!-- cycle detected -->"))
		         ^
xmltree/marshal.go:99:13: Error return value of `e.w.Write` is not checked (errcheck)
			e.w.Write(el.Content)
			         ^
xmltree/marshal.go:139:18: Error return value of `io.WriteString` is not checked (errcheck)
			io.WriteString(e.w, e.indent)
			              ^
xmltree/marshal.go:151:18: Error return value of `io.WriteString` is not checked (errcheck)
			io.WriteString(e.w, "\n")
			              ^
xmltree/marshal.go:161:19: Error return value of `io.WriteString` is not checked (errcheck)
				io.WriteString(e.w, e.indent)
				              ^
xmltree/xmltree.go:330:10: Error return value of `el.walk` is not checked (errcheck)
		el.walk(search)
		       ^
xmltree/xmltree.go:332:11: Error return value of `root.walk` is not checked (errcheck)
	root.walk(search)
	         ^
xmltree/xmltree_test.go:164:9: Error return value of `el.walk` is not checked (errcheck)
	el.walk(func(el *Element) {
	       ^
xmltree/xmltree_test.go:165:10: Error return value of `el.walk` is not checked (errcheck)
		el.walk(func(el *Element) {
		       ^
xsdgen/cli.go:102:10: Error return value of `fs.Parse` is not checked (errcheck)
	fs.Parse(arguments)
	        ^
wsdlgen/cli.go:42:10: Error return value of `fs.Parse` is not checked (errcheck)
	fs.Parse(arguments)
	        ^
wsdlgen/wsdlgen.go:309:16: composites: `encoding/xml.Name` composite literal uses unkeyed fields (govet)
			XMLName:    xml.Name{p.wsdl.TargetNS, part.Name},
			            ^
wsdlgen/wsdlgen.go:331:13: composites: `encoding/xml.Name` composite literal uses unkeyed fields (govet)
			XMLName: xml.Name{p.wsdl.TargetNS, part.Name},
			         ^
xmltree/xmltree.go:134:9: composites: `encoding/xml.Name` composite literal uses unkeyed fields (govet)
	return xml.Name{defaultns, qname}
	       ^
xmltree/example_test.go:228:1: tests: ExampleMarshalNested refers to unknown identifier: MarshalNested (govet)
func ExampleMarshalNested() {
^
xsd/xsd.go:303:2: unreachable: unreachable code (govet)
	return xml.Name{}
	^
xmltree/xmltree_test.go:284:3: SA4004: the surrounding loop is unconditionally terminated (staticcheck)
		break
		^
xsdgen/config.go:488:8: const `soapenc` is unused (unused)