Create named UnspecifiedError for misc. errors

Use for error cases that are not typical occurrences. This will allow consistent formatting of all wskdeploy errors and avoid in-line error strings where possible.

This would be used wherever we use the generic constructor:
```
errors.New()
```

Such as in filesystemreader. go:
```
return "", nil, errors.New("Unsupported action type.")

return errors.New("Conflict detected for action named " + existAction.Action.Name + ". Found two locations for source file: " + existAction.Filepath + " and " + fileAction.Filepath)
```