Some initial error handling for DEM/DTM

[kbrandwijk]

Tweaked DEM/DTM error handling a bit as per our discussion:

• Display user-facing error string to the user
• Don't swallow DEM processing errors, but log and rethrow instead
• Add initial error handling to some DTM providers:
  • Check if API key is provided for Finland
  • Check if token is provided for Denmark
• Refactor extents logic to allow for multiple extents to define a DTM provider and add correct ones for France

[iwatkot]

@kbrandwijk, I recommend removing the webui/generator from the PR since you have like three symbols changed there. But you can go the hard way and merge the main. 🤣

[kbrandwijk]

Rebased