Improve errors in management console

Created by: chrismwendt

New error messages (previously it said error: 500):

• I switched from ajax to fetch in order to read the body of a non-200 response
• The server now sends back structured JSON responses with error and code fields on error @slimsag Any other UI I should check to make sure this didn't break anything?

Resolves https://github.com/sourcegraph/sourcegraph/issues/1821