Provide better error messages when connection fails

[clhenrick]

:beetle: If bug

• [ ] Operating System and version:
• [ ] Browser and version:
• [ ] Steps to reproduce (including dashboard link):
• [x] Description of issue:

When the browser connector fails to connect to a MapD instance, there often is no useful error message in the console. Instead something like this will often show up:

Nothing is visible in the network response either.

It would improve the developer friendliness of this library immensely if we could provide useful error messages.

:new: If feature request

• [x] User story: As an application or data visualization engineer using mapd-core and/or mapd backend rendering, I need to be able to debug mapd-connector when the connection fails.
• [ ] Specific requirements/functionality:
• [ ] Mocks (if applicable):

cc @alexbaden @andrewseidl I know there was some related issue about CORS somewhere, maybe you might have some ideas for fixing this as well.

[nermaljcat]

Look at: https://github.com/omnisci/mapd-connector/blob/master/thrift/browser/thrift.js#L336-L346

Seems the Thrift.Transport assumes success...

[jonvuri]

@nermaljcat is correct, this is an issue with Thrift not handling the .onerror event of the XHR, resulting in the exception blowing up the stack with no info.

[jonvuri]

Tracking in https://jira.omnisci.com/browse/FE-8235