Fix the compatibility Cause() API. (cockroachdb#17) Fix the compatibility Cause() API.

safe details/messages: clarify cases with empty formats (cockroachdb#3) safe details/messages: clarify cases with empty formats

Export two types at the top level. 

Fix the top level export of FormatError. 

Implement mising FormatError methods ... for assertion failures and unimplemented errors. Additionally, test the `%+v` formatting of `errors.Newf()`, `NewAssertionErrorWithWrappedErrf()`.

Add a clarifying comment. 

Add explanatory comments for visibility of error fields. 

Update the stack parser to the latest raven-go code. 

Fix a call stack format mismatch between go 1.11 and 1.12 

Import the initial implementation.