Fix issues raised by go vet

[mordyovits]

Fixed a conflation of types between a filename and the *config.Config (source
and target). The fmt was using a %s and meant the filename but used the nil
*config.Config

Removed an erroneous strconv.Itoa. The fmt was using %d anyway.

[robfig]

fmt.Errorf

[mordyovits]

I'll change it to that when I get a chance.

[robfig]

I don't get it -- what does renaming to *_fn fix? (Also, Go uses lowerCamel, not snake_case)

[mordyovits]

The problem is in line 367:
target, error := ReadDefault(target) <-- notice how target changed types
if error != nil {
t.Fatalf("Unable to read target config file '%s'", target) <-- but is then used as if it's a string (filename)

So my patch makes clear what's a *Config and what's a string. Without my patch, whenever line 367 calls Fatalf(), what it prints is nil, not the intended filename.

The same problem was with source.

[mordyovits]

Ok, fixed all the issues you raised, and it passes go test and go vet is still clean.

[robfig]

Thanks!