Handle 403 error

[ksauzz]

We sometimes observed https://api.materialsproject.org/heartbeat responds a 403 error, then mp-api just raises JSONDecodeError.
It would be nice to show users a 403 error, and what could happen such as rate limit or something.

JSONDecodeError: Expecting value: line 1 column 1 (char 0)

refs:
https://matsci.org/t/pymatgen-ext-matproj-mpresterror-rest-query-returned-with-error-status-code-403-content-berror-code-1020/47992
https://matsci.org/t/mpresterror-rest-query-returned-with-error-status-code-403/792

[munrojm]

Yes, I agree with this. The client should communicate better when the API service appears to be unreachable.

[esoteric-ephemera]

Should be fixed in #1053: Tf the heartbeat page returns a 403, emit a warning linking to the status page, and the client should work as normal