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.