Update exception class name.

As suggested by aspiers in https://github.com/mnalis/maildir-deduplicate/commit/438904af5ed30c3b8aa87c7d79b2cf3f0d678e28#commitcomment-8304329
Safer datetime parsing.

As suggested by @aspiers. See:
https://github.com/mnalis/maildir-deduplicate/commit/c87818c0b47b779ee5ea6eaf6929047ce811b791#commitcomment-8304336
Whitspaces.