Evaluate error string lazily for performance.

[noamkush]

While profiling our test suite, I noticed a significant amount of time is spent in various __repr__ methods in factory_boy. Most of these calls come from Resolver.__getattr__ which uses them to generate pretty error messages. While it's nice to have user friendly error messages, this is frequently called from deepgetattr which dismisses the error immediately. Therefore, the actual string may be lazily evaluated only when necessary. This simple change significantly improves our test suite run time.