Improve Error Handling and Reporting

[WillNilges]

Opening this issue to keep track of all the error-related Issues.

• No error message when unable to access/mount ISOs #12
• No error message when unable to start VMs #13
• psycopg2 closes connection after a while #138
• RQ worker fails silently on frontend when config file not present #139
• Database access component fails silently #152