Don't print "No global of this name exists in this module." on UndefValError

[LilithHafner]

When we get an UndefValError, we know !isdefined(Main, :a). This is highly correlated with "No global of this name exists in this module." Consequently, the second sentence in the suggestion

julia> a
ERROR: UndefVarError: `a` not defined in `Main`
Suggestion: check for spelling errors or missing imports. No global of this name exists in this module.

Carries less than one bit of information. I don't like showing users hints that carry less than one bit per sentence. In the cases where a global does exist, that sentence carries much more information and is worth keeping. In any event, a super power user can infer from the lack of that sentence that no global exists.

Master

julia> function f()
           global a = 2
       end
f (generic function with 1 method)

julia> a
ERROR: UndefVarError: `a` not defined in `Main`
Suggestion: add an appropriate import or assignment. This global was declared but not assigned.

julia> b
ERROR: UndefVarError: `b` not defined in `Main`
Suggestion: check for spelling errors or missing imports. No global of this name exists in this module.

PR

julia> function f()
           global a = 2
       end
f (generic function with 1 method)

julia> a
ERROR: UndefVarError: `a` not defined in `Main`
Suggestion: add an appropriate import or assignment. This global was declared but not assigned.

julia> b
ERROR: UndefVarError: `b` not defined in `Main`
Suggestion: check for spelling errors or missing imports.

Fixes #52273 (cc @vtjnash @jishnub)