Convert PROCESS error handling to new system

[timothy-nunn]

There is a new exception handling system in PROCESS, process.exceptions which (currently) contains three error types:

• ProcessError: the base PROCESS error.
• ProcessValidationError: errors in validation of input.
• ProcessValueError: ValueError's raised within PROCESS.

PROCESS needs to be upgraded to this system in two ways:

1. All of the level 3 errors in process/utilities/errorlist.json that are created from inside Python should become an appropriate exception, this may require adding additional exceptions to process.exceptions.
2. All exceptions currently raised in Python should raise a PROCESS-specific error. This allows us to isolate errors originating from PROCESS models and those from other packages. Similarly, any dependant of us will be able to distinguish PROCESS errors.