Gracefully Handle Tile Server Errors

[pjdufour]

First...

• I've made sure there are no duplicate issues

Issue type

• bug report
• feature request

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If this is a bug report (if not, please remove this section)

Description

When railgun returns and error from the server, including one with a valid JSON response body, the following error occurs on the client.

map.component.js:366 Uncaught TypeError: Cannot read property 'forEach' of undefined
    at XMLHttpRequest.r.onload (map.component.js:366)

Expected behavior

If the response code is not equal to 200, then log the error body to the console. If the error body if valid JSON, then log the parsed body.

If the response code is equal to 200, check that the response body includes a features property before attempting to do forEach on the array.

Current behavior

See description.

Steps to reproduce

Corrupt railgun config.

Environment

N.A.

Device:

N.A.

Browser:

N.A.

Additional helpful info/screenshots

None