Improve error messages so users can identify the actual problem

E.g.

```
SpringTernary: Ternary operation missing parentheses.
```

is a bit terse. I had to look at the source code to find that actually it means that the *test* has to be in parentheses.

Another one:

```
SpringHeader: header.mismatch
```

That's all you get. No idea what that means. UPDATE: it turned out to be an "http" URL in the license link.