make/buildtest: print errors

Display all stderr output but suppress warnings.

implements https://github.com/RIOT-OS/RIOT/issues/835
include warnings

addresses https://github.com/RIOT-OS/RIOT/pull/883#issuecomment-37833467
Merge pull request #883 from LudwigOrtmann/issue_835_b

make/buildtest: print errors (alternative to #836)