Use __str__, not __repr__, for human readable provider error text

[benclifford]

See PR #1966 for more context

Changed Behaviour

Any error reporting which uses the repr of an exception rather than the str of an exception will now look more ugly, as the human readable text will now only be available in str, not in repr. However, Python stack trace information is printed using str so at least that style of error reporting should not change here.

Type of change

• Update to human readable text: Documentation/error messages/comments
• Code maintentance/cleanup