OnDemand: Add 'Troubleshoot' section

[HenrikBengtsson]

Add a 'Troubleshoot' section to https://www.c4.ucsf.edu/howto/ondemand.html with information on how to access OnDemand session log output, how to report errors (e.g. include log output), ...

[hgputnam]

Good idea, I find myself typing the same sentences on Slack quite a few times...