Discovered in #2466197: Staging directory should not have to be writeable.
Problem/Motivation
config_get_config_directory() just throws a \Exception when the config directory type does not exist - that's way too generic.
Proposed resolution
Create a more specific exception
Remaining tasks
User interface changes
None
API changes
A new exception - but any code catching the existing exception will work because the new exception will extend (at some point) from \Exception