Properly print warnings to stderr. Also, use : instead of 'echo' as a
fallback as the latter spits out garbage to stdout.
Examples without pep8 installed.
Before:
$ ./hooks/pre-commit
./hooks/pre-commit: line 97: WARNING:: command not found
ERROR: pep8 tests failed!
After:
$ ./hooks/pre-commit
WARNING: pep8 is not installed, using dummy placeholder!