Make loading function throw exceptions, not show messages.

Refs #7955
Do updates only when Dead Timt Table files where found.

Refs #7955
Move error reporting code to one place.

Plus make error message explicit about the fact that
no dead time correction was applied.

Refs #7955
Check if Dead Time file is valid before using it.

Refs #7955
Merge remote-tracking branch 'origin/bugfix/7955_muon_deadtime_file_logic'