fix mercurial step

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

bin/steps/mercurial

 #!/usr/bin/env bash
 
 # Install Mercurial if it appears to be required.
-if (grep -Fiq "hg+" requirements.txt) then
+if [[ -f "requirements.txt" ]]; then
+	if (grep -Fiq "hg+" requirements.txt) then
 		/app/.heroku/python/bin/pip install  mercurial | cleanup | indent
+	fi
 fi
+

vendor/pip-pop/pip-grep

@@ -8,6 +8,7 @@ Options:
   -h --help     Show this screen.
 """
 import os
+import sys
 from docopt import docopt
 from pip.req import parse_requirements
 from pip.index import PackageFinder
@@ -73,5 +74,5 @@ if __name__ == '__main__':
     try:
         main()
     except Exception:
-        pass
+        sys.exit(1)