Common error hadling

[paul121]

Working on #202 we realized it would be nice to have a common way to surface errors from the map to the user. We should look at OL examples and/or new features in OL v10 that might make this easier.

Use cases:

• Errors when snapshotting (see Add snapshot control to download and print map #202)
• Errors when requesting map tiles
• ... other core map behaviors?