Error and Exception handling needs fully designing

[hayfield]

At present there are rudimentary attempts at error handling (and logging of what has happened). This needs improving to work more generally.

Current systems:

• Exception objects with custom strings
• Exception objects with no additional information
• A base concept of iati.exceptions
• ValidationErrorLog (for validation errors) - should this model be used elsewhere?

[allthatilk]

Think the names of existing errors also needs a look at as part of this as err-not-iati-xml-missing-required-element is not fully reflective of all contexts in which it is raised.

[hayfield]

Note: They're Validation errors, this issue is about the Python-language concept of Exceptions (dealt with via try-except.

...or at least it should be - looks like the last bullet needs re-wording!