Split long errors in several lines (#491)

Split long errors in several lines (#491)

* Pretty print long errors in several lines

This is done by using newline characters to insert a new error prefix. A couple
of new facilities to encourage splitting of long errors are defined:
Errors.Wrap and Errors.Set (Wrapping_Message, Wrapped_Exception)

* Rename Last_Chance_Handler to Alr.Last_Chance_...

To avoid possible conflicts with other libraries and follow the naming of
other packages.

* Fix testsuite output matches on new line breaks

* Code review fixes