Include full path in error messages about nonexistent files (e.g. when loading a workspace)

[confluence]

At the moment if a workspace includes a file which is moved or deleted, when the workspace is loaded the error message about this file only contains the filename, not the full path to the file. This is unhelpful if the user has lots of files which have the same name but are in different subdirectories (this has actually come up recently, and seems like a common use case).

The error message should include the full path to the file. If possible (if it's not considered a security risk¹) we should ensure that it's the correct absolute filesystem path in the case where the CARTA root is not the filesystem root. The user should be able to copy this path and use it directly for debugging.

1. My understanding is that we use the CARTA root mostly to avoid showing the user irrelevant system directories, and that we don't rely on it for security, so exposing the real filesystem path to the CARTA root should not be a problem.