feat(www): enable providers to send custom error responses via the dynamic key

[daniellacosse]

This PR enables service providers to return an error object to inform the end user about something wrong with their dynamic key or server connection. Service providers could then volunteer contact information via this error message.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 32%. Comparing base (8ec1aae) to head (cbcf94c).

Files	Patch %	Lines
...line_server_repository/access_key_serialization.ts	71%	2 Missing ⚠️
src/www/model/errors.ts	33%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1830   +/-   ##
======================================
- Coverage      32%     32%   -1%     
======================================
  Files          45      45           
  Lines        2610    2620   +10     
  Branches      337     340    +3     
======================================
+ Hits          859     860    +1     
- Misses       1751    1760    +9     

Flag	Coverage Δ
apple	15% <ø> (ø)
ios	15% <ø> (ø)
maccatalyst	15% <ø> (ø)
macos	15% <ø> (ø)
unittests	32% <60%> (-1%)	⬇️
www	40% <60%> (-1%)	⬇️

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[fortuna]

We need to better understand the trade-offs between the different approaches, which also included a client response and a Client API.
During planning, we decided we needed a document for discussion.

There are also open questions regarding what format we should use, and what the UI should look like, but they can be decoupled from the chosen approach.

[fortuna]

This is awesome, and it will be super helpful to providers. Thanks for implementing it!

It's ok to merge, but please test the functionality to make sure it's working end-to-end after all the changes.

[fortuna]

We also need to think about communicating the new feature in the next release, and perhaps add that to the wiki.