Expose x-error-message header via error_message property for 500 code

[glensc]

This gives 500 error right now:

• https://api-v2launch.trakt.tv/shows/game-of-thrones?extended=full

HTTP/2 500
date: Mon, 12 Dec 2022 05:38:15 GMT
content-type: text/html
x-frame-options: SAMEORIGIN
x-xss-protection: 1; mode=block
x-content-type-options: nosniff
x-download-options: noopen
x-permitted-cross-domain-policies: none
referrer-policy: strict-origin-when-cross-origin
cache-control: no-cache, no-store
x-error-message: Internal server error. Please open a support issue and include your full API request and response, including all headers.
vary: Accept-Encoding
x-request-id: cbb03120-fdbe-46ba-b7e9-1311c7359474
x-runtime: 0.004596
cf-cache-status: BYPASS
server: cloudflare
cf-ray: 77841cdccebac7f7-TLL

expose the x-error-message header value via e.error_message.

[glensc]

such header could be missing too:

HTTP/2 503
date: Mon, 12 Dec 2022 09:12:55 GMT
content-type: application/json; charset=utf-8
content-length: 0
cf-cache-status: BYPASS
server: cloudflare
cf-ray: 778557536afdc7de-TLL

EDIT: this is 503 code not 500