Implementing a sanity check for all positions

[CraignRush]

Apparently, for some users problems occur while saving their session or starting the lamella milling job.

A sanity check before doing these things could resolve this issue. P.e. it could include:

• checking the number of patterns per alignment image
• looking for empty or mismatching pattern entries
• Removing falsely loaded images from the saving process
• Displaying a more detailed error description in the log panel