confusing exceptions thrown when a StringDocumentSource is passed in but
parsers for text formats fail.

The RIO binary formats will fail on StringDocumentSource because no
input streams can be found. This looks like an IO error, that stops
parsers from being attempted and gives the final user an IO error,
without parsing errors reports.

Better matching of parseability of a source and parser to use would be
useful. In the meantime, a special catch for the binary formats should
do.
added a getFormat method to avoid fetching around a manager just for
this purpose
Test cases improved. 

This test builds a variety of axioms, to be reused for testing other
visitors