Map change errors/failures should be specified and passed through to platform SDKs

[ericrwolfe]

Currently map.cpp only returns single mbgl::MapChangeDidFailLoadingMap event for any and all map failures, making it impossible for a developer to distinguish between map failures.

For example, on iOS the same error is returned whether a map load failed due to a bad access token, a bad style, or a bad network connection.

/cc @1ec5

[1ec5]

The error was implemented in #3063 to use a generic code and nil message because of this issue. As such, it’s debatable whether we’ve actually fixed #2762, and certainly this blocks #2775. mbgl notifications could be structs with richer information, but at that point we might as well start throwing real C++ exceptions and catching them at the SDK level, along the lines of #3063.

[1ec5]

This is happening in #8418.

[boundsj]

#8418 landed in the master branch.