Fail gracefully on undecodable install output.

testing/resources/not_installable_repo/setup.py

@@ -0,0 +1,17 @@
+# -*- coding: utf-8 -*-
+from __future__ import print_function
+from __future__ import unicode_literals
+
+import sys
+
+
+def main():
+    # Intentionally write mixed encoding to the output.  This should not crash
+    # pre-commit and should write bytes to the output.
+    sys.stderr.write('☃'.encode('UTF-8') + '²'.encode('latin1') + b'\n')
+    # Return 1 to indicate failures
+    return 1
+
+
+if __name__ == '__main__':
+    exit(main())