Add error_handler and use it.

pre_commit/error_handler.py

@@ -0,0 +1,42 @@
+from __future__ import absolute_import
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import contextlib
+import io
+import os.path
+import traceback
+
+from pre_commit.errors import FatalError
+from pre_commit.store import Store
+
+
+# For testing purposes
+class PreCommitSystemExit(SystemExit):
+    pass
+
+
+def _log_and_exit(msg, exc, formatted, print_fn=print):
+    error_msg = '{0}: {1}: {2}'.format(msg, type(exc).__name__, exc)
+    print_fn(error_msg)
+    print_fn('Check the log at ~/.pre-commit/pre-commit.log')
+    store = Store()
+    store.require_created()
+    with io.open(os.path.join(store.directory, 'pre-commit.log'), 'w') as log:
+        log.write(error_msg + '\n')
+        log.write(formatted + '\n')
+    raise PreCommitSystemExit(1)
+
+
+@contextlib.contextmanager
+def error_handler():
+    try:
+        yield
+    except FatalError as e:
+        _log_and_exit('An error has occurred', e, traceback.format_exc())
+    except Exception as e:
+        _log_and_exit(
+            'An unexpected error has occurred',
+            e,
+            traceback.format_exc(),
+        )