Fail gracefully on undecodable install output.

pre_commit/error_handler.py

@@ -18,8 +18,19 @@ class PreCommitSystemExit(SystemExit):
     pass
 
 
+def _to_bytes(exc):
+    try:
+        return bytes(exc)
+    except Exception:
+        return five.text(exc).encode('UTF-8')
+
+
 def _log_and_exit(msg, exc, formatted, write_fn=sys_stdout_write_wrapper):
-    error_msg = '{0}: {1}: {2}\n'.format(msg, type(exc).__name__, exc)
+    error_msg = b''.join((
+        five.to_bytes(msg), b': ',
+        five.to_bytes(type(exc).__name__), b': ',
+        _to_bytes(exc), b'\n',
+    ))
     write_fn(error_msg)
     write_fn('Check the log at ~/.pre-commit/pre-commit.log\n')
     store = Store()