Lower severity of source-symbol-not-found diagnostic

[d-ronnqvist]

Bug/issue #, if applicable:

Summary

This PR makes two changes to the diagnostic when the source symbol of a symbol graph relationship can't be found:

• Elaborate in the summary and explanation to clarify that this is an issue with the tool that created the symbol graph and not something that the developer can resolve themselves.
• Lower the severity of this diagnostic so that this issue doesn't fail the build.

Since the solution to this issue is for the tool that created the symbol graph file to not include this relationship (or move it to a different file), it's safe for DocC to proceed without processing this symbol graph relationship.

Dependencies

None.

Testing

• Manually edit a symbol graph file to add a relationship where the "source" isn't a symbol ID for a symbol defined in that symbol graph file.
• Build documentation for that symbol graph file.
• There should be a warning, not an error, about the incorrect symbol graph relationship.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [x] ~Added~ Updated tests
• [x] Ran the ./bin/test script and it succeeded
• [x] ~Updated documentation if necessary~ Not applicable

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

After an offline discussion about this PR I've further reduced the severity to a debug-only assertion in https://github.com/apple/swift-docc/pull/966/commits/2e64b4be80cd5232f6d62d963ac265eb3514ecec.

[d-ronnqvist]

@swift-ci please test