OnCall plugin configuration page and api-key issue

[raphael-batte]

When user configures the onCall plugin on the */plugins/grafana-oncall-app page, in case of a connection error (for example host was invalid), the api-key becomes outdated, but the user does not receive this information as error message.

Proposal:

• add to error message information about key and hint what user should generate new key
• add hint to documentation