funccount: Bail early if there are no matching functions (iovisor#792)

funccount now bails early with an error if there are no
functions matching the specified pattern (the same applies
to tracepoints and USDT probes). For example:

```
No functions matched by pattern ^sched:sched_fork$
```

Fixes iovisor#789.

tools/funccount.py

@@ -107,10 +107,6 @@ def attach(self):
  elif self.type == "u":
  pass # Nothing to do -- attach already happened in `load`
 
- if self.matched == 0:
- raise Exception("No functions matched by pattern %s" %
- self.pattern)
-
  def _add_function(self, template, probe_name):
  new_func = "trace_count_%d" % self.matched
  text = template.replace("PROBE_FUNCTION", new_func)
@@ -199,6 +195,10 @@ def load(self):
  if debug:
  print(bpf_text)
 
+ if self.matched == 0:
+ raise Exception("No functions matched by pattern %s" %
+ self.pattern)
+
  self.bpf = BPF(text=bpf_text,
  usdt_contexts=[self.usdt] if self.usdt else [])
  self.clear() # Initialize all array items to zero