Show in the browser compilation errors only

By filtering SBT "problems" based on their severity, we can tell
Play to show in the browser compilation errors only.

Thanks to this fix, if the compiler indicates a warning followed
by an exception, the developer won't see the warning instead of
the exception in his browser.

Thanks to @baloo for his help.
Merge pull request #950 from Dinduks/fix_playreloader_bug

Show in the browser compilation errors only