[build_exception] Decode build failure messages

This is so that the test harness pretty-prints build error messages in
trace mode, instead of dumping a raw python bytes object.

llvm-svn: 369987

lldb/packages/Python/lldbsuite/test_event/build_exception.py

@@ -13,4 +13,4 @@ def __str__(self):
  @staticmethod
  def format_build_error(command, command_output):
  return "Error when building test subject.\n\nBuild Command:\n{}\n\nBuild Command Output:\n{}".format(
- command, command_output)
+ command, command_output.decode("utf-8"))