Fixed #20604 - Fixed file example in docs.

Thanks Chris Lasher for the report.
Fixed #20599 -- Changed wording of ValueError raised by _load_library

The _load_library method on BasePasswordHasher turns ImportErrors
into ValueErrors, this masks ImportErrors in the algorithm library.
Changed it to a clearer worded error message that includes
the ImportError string.
Worked around Python 3.3 modified exception repr

Refs #20599.
Fixed #20606 -- Fixed 'for' example in template tag docs

Thanks batisteo for the report.
Adjusted the docs to reflect when the checksetup command was added.