[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 @@ class BuildError(Exception):
     @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"))