* [mtouch][mmp] Share error.cs and make it safer against FormatException
Beside the obvious code sharing [1] this makes the error reporting code
safer against `FormatException`.
Even with code review this happened a few times in the past - we get
a string that can't format correctly in some error handling code. This
means the `FormatException` must be fixed before we can see the
underlying root issue (delaying bug fixes and support).
Recent changes made for localization makes it harder to review the
original strings (and even harder for translated strings) for
correctness. This means we can end up with something like
```
String.Format ("{0} {1}", "foo");
``
and easily miss it (since they split between different files) if there
are several changes inside a PR.
Finally sprinkle some unit tests :)
[1] we should move everything to use `ProductException` but that will be
for another PR...
2be35fb1c8