better processing errors reporting

[rngadam]

errors with (for example) converting slides and processing the speaker notes should be reported and displayed in the sidebar:

• error should be stored in spreadsheet?
• or perhaps Logger.log retrieved and displayed?

[andiechu]

Add error handling solution to Onboarding procedure, which can be reused to all the system in the future:

1. send error emails to the script runner
2. show a dialog in the spreadsheet window to warn about the error
3. record the error messages into a spreadsheet: members share -> Coderbunker Systems -> logs -> Coderbunker Scripts Logs - Onboarding (4. also all errors have already logged into Google Stackdriver Logging system, using console.error/info/warn())

See commits here: https://github.com/coderbunker/gas/pull/55/commits/5b61aac5d78d82f57bafb41c73d0c9ca929ef853