Improve failure message for pre-commit. Closes #36

pre_commit/resources/pre-commit.sh

@@ -1,3 +1,9 @@
 #!/usr/bin/env bash
 
-pre-commit
+which pre-commit > /dev/null
+if [ $? -ne 0 ]; then
+    echo '`pre-commit` not found.  Did you forget to activate your virtualenv?'
+    exit 1
+fi
+
+pre-commit