Better error handling when adding malformed GeoJSON

[noahmoss]

Describe the bug
When adding GeoJSON for a custom map that is malformed JSON or GeoJSON (to populate the identifier/display name dropdowns), the modal is closed and a stack trace is displayed in the bottom right of the screen. This is similar to #14635 but is a different underlying problem, and won't be resolved by my fix to that issue.

To Reproduce
Steps to reproduce the behavior:

1. Go to Admin -> Settings -> Maps
2. Click on 'Add a map'
3. Type in an arbitrary name
4. Add a URL pointing to malformed GeoJSON. The test file we're currently using for our backend tests actually triggers this, since it's missing a top-level features field: https://raw.githubusercontent.com/metabase/metabase/master/test_resources/test.geojson

Expected behavior
An sane error message should be shown instead of the stack trace

Screenshots

geojson-error.mov

Information about your Metabase Installation:

Current master branch on Firefox and MacOS

[flamber]

On pre-39 it would just create an unhelpful error, which is closer to a P3, but the "Something went wrong" makes it a P2

[noahmoss]

This also happens if you put in a URL like https://metabase.com which passes backend validation but isn't JSON.