Adding contributions through the bot when existing contributions do not validate lead to "trouble processing request"

:warning: This is a guess diagnostic and would appreciate input from maintainers of the all-contributions bot.

**Describe the bug**

Adding contributions through `@all-contributions` fails when existing contributions include authors not listed as committers.

**To Reproduce**
Steps to reproduce the behavior:

1. Using the CLI, add a `code` contribution for a GitHub user that hasn't contributed to the history of the repository
2. Push to GitHub
3. Try to use `@all-contributions please add ...` on that repository
4. The `We had trouble processing your request` error is likely to appear

**Expected behavior**
A clearer error message with some diagnostic information would be appreciated but in this case, not failing would be reasonable as well.
Consistency between `CLI` and bot behavior.

**Screenshots**

See [this issue](https://github.com/unode/firefox_decrypt/issues/97#issuecomment-1652448692) where the request was made and the subsequent failure message.

At the same time, using the CLI, the output of `all-contributors check` was:
```
$ yarn all-contributors check
yarn run v1.22.19
$ (...)/firefox_decrypt/node_modules/.bin/all-contributors check
Unknown contributors found in .all-contributorsrc:
tennox, Anthropohedron, alejandro-amo, yermulnik
```
but adding credit to `rma-x` locally [worked fine](https://github.com/unode/firefox_decrypt/commit/d2ddb6586f9d67de9e72f7bcbb54ebee394aacff).

**Additional context**

While no co-authored commit exists for some of these users, [they have been acknowledged in commit messages](https://github.com/unode/firefox_decrypt/commit/3b6f6e1dcd1f03388cc48fc9280749b20f7a51eb)

If my diagnostic is correct, it would be useful to have a mechanism to override this type of validation.
In this case credit is due but the bot can't validate that `code` contributions exist if a commit wasn't made.
Would having a `skipCheck` or adequately named flag at the user level be fitting?

I also couldn't find much explanation for the "trouble processing request" error. Would this be a candidate for a FAQ?