produce better context information for error cases in __main__

[marc-portier]

We already have a log.exception() used in this case but this only shows the stack-trace in pysbuyt code that led to the error.

For pysubyt users though the more relevant information is:

• which line of the template
• which line of the processed data

to that extend we should be able to

• better trace current stage of processing --> see also #28
• introspect jinja upon error to check if it can produce template processing details at runtime (what line in what file?)

[marc-portier]

see also #34