Include newlines in StatementError formatting

Include newlines in StatementError formatting

Revised the formatting for :class:`.StatementError` when stringified. Each
error detail is broken up over multiple newlines instead of spaced out on a
single line.  Additionally, the SQL representation now stringifies the SQL
statement rather than using ``repr()``, so that newlines are rendered as is.
Pull request courtesy Nate Clark.

Fixes: #4500
Closes: #4501
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4501
Pull-request-sha: 60cc0ee68dc96b8f483a60d37bcb26b6c6d53efe

Change-Id: I79d8418b7495e5691c9a56f41e79495c26a967ff