Caspar: Raise error on unsupported multi-camera rig

[tordnat]

Address #4376

[gemini-code-assist]

Code Review

This pull request updates the error handling in the CasparBundleAdjuster by changing a warning log to a fatal exception when encountering a non-ref rig camera with a variable pose. The reviewer pointed out that the log message should be updated to remove the phrase "Observations skipped," as it is misleading now that the process terminates instead of continuing.

[B1ueber2y]

Similarly applied to the other warnings in the implementation like https://github.com/colmap/colmap/blob/main/src/colmap/estimators/bundle_adjustment_caspar.cc#L247, where I think we should also throw.

Thanks. Can you also address this?

[B1ueber2y]

Not sure if there are other such cases.