Better (user) error messages

[Kerbalnut]

In the latest update to the error messages returned if PDF file can't be found, while now they give more description, the information may not always be relevant to the specific error that triggered it. (Most are copy-and-pasted).

More testing on which specific scenarios trigger which error events, and analysis done on what the user (User-Model) most likely intended or expected in each situation.

This will enable more concise, more accurate error messages to be written.