HTML errors from API requests display HTML in the alert

[kevinansfield]

Issue Summary

When an API request fails and a service between the client and the Ghost service (e.g. a proxy/CDN returning a timeout screen) returns HTML, that content gets displayed in the error alert rather than a sensible message.

Steps to Reproduce

T.B.D

Technical details:

• Ghost Version: LTS (possibly master, needs replication)

[cobbspur]

FYI: Ran into a similar issue today when trying to perform a password reset from email (LTS).