Document how to identify the common things that have gone wrong when LDMX-sw segfaults

[EinarElen]

In particular OTTOMH

• Looking for error messages outside of the big ROOT printout (and in the future, how to disable the ROOT printout!)
• Common exceptions like
  • Wrong type for one of the parameters (float vs double etc)
  • Missing condition objects
• p.pause()
• As a last resort, a step by step description of how to run your program with GDB and get a stack trace of the actual place that the issue happened at