errorlib: stack analysis improvements

• Adds "p_pw" pattern in the list of variables to hide when doing frame analysis. (closes #1126)

• Fixes wrong usage of sys.exc_info that some times might have lead to analysing the wrong stack.

• Introduces explicit frame.f_locals dereferencing into a local variable. http://utcc.utoronto.ca/~cks/space/blog/python/FLocalsAndTraceFunctions

• Fixes import statement in inveniocfg loading of regression tests that was confusing errorlib stack analysis.