The idea is that the parse_config method of extension modules can raise either a ConfigError or a JsonValidationError,
and it will be magically turned into a legible error message. There's a few components to it:
* Separating the "path" and the "message" parts of a ConfigError, so that we can fiddle with the path bit to turn it
into an absolute path.
* Generally improving the way ConfigErrors get printed.
* Passing in the config path to load_module so that it can wrap any exceptions that get caught appropriately.
Fixes a bug where the default attribute maps were prioritised over
user-specified ones, resulting in incorrect mappings.
The problem is that if you call SPConfig.load() multiple times, it adds new
attribute mappers to a list. So by calling it with the default config first,
and then the user-specified config, we would always get the default mappers
before the user-specified mappers.
To solve this, let's merge the config dicts first, and then pass them to
SPConfig.
