fix: Missing external configuration error message

[kuipumu]

Description

This PR adds a fix for the LTI external configurations feature issue https://github.com/openedx/xblock-lti-consumer/issues/420

Type of Change

• [x] Modify _get_lti_config_for_block function on plugin/api.py module to raise an ExternalConfigurationNotFound error when no external LTI configuration is found.

Testing

1. Create a course and add a LTI consumer XBlock.
2. Edit the XBlock settings and set "Configuration Type" to "Reusable Configuration".
3. Edit the XBlock settings and set "LTI Reusable Configuration ID" to an nonexistent external configuration (Ex: test:test).
4. Save the XBlock and reload the studio editor.
5. The studio view should show an exception.
6. Go to the learning MFE and load the unit where the XBlock is.
7. The learning MFE should show an general unexpected exception.

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[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f36725a) 97.95% compared to head (291567b) 97.93%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #423 +/- ## ========================================== - Coverage 97.95% 97.93% -0.02% ========================================== Files 77 77 Lines 6686 6689 +3 ========================================== + Hits 6549 6551 +2 - Misses 137 138 +1 ``` | [Flag](https://app.codecov.io/gh/openedx/xblock-lti-consumer/pull/423/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/openedx/xblock-lti-consumer/pull/423/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx) | `97.93% <75.00%> (-0.02%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files](https://app.codecov.io/gh/openedx/xblock-lti-consumer/pull/423?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx) | Coverage Δ | | |---|---|---| | [lti\_consumer/api.py](https://app.codecov.io/gh/openedx/xblock-lti-consumer/pull/423?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx#diff-bHRpX2NvbnN1bWVyL2FwaS5weQ==) | `97.91% <75.00%> (-1.01%)` | :arrow_down: |

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[mphilbrick211]

Hi @kuipumu! Would you mind rebasing this pull request? Doing so might help resolve the codecov errors. If that doesn't work, I'll look into it for you. Thanks!

[mphilbrick211]

Hi @kuipumu! Would you mind rebasing this pull request? Doing so might help resolve the codecov errors. If that doesn't work, I'll look into it for you. Thanks!

Hi @kiram15! Friendly follow-up on this!

[mphilbrick211]

Hi @kuipumu! Just checking in to see if you plan to pursue this pull request?

[mphilbrick211]

Hi @kuipumu! I'm going to close this pull request for now, but we can reopen in the future if you'd like to pursue. Thanks!

[openedx-webhooks]

@kuipumu Even though your pull request wasn’t merged, please take a moment to answer a two question survey so we can improve your experience in the future.