rez-gui cannot open contexts with missing package(s)

When using rez-gui to update .rxt files, if the file contains a reference to a package that cannot be found on disk, the context cannot be opened. This makes it difficult to fix because the context either needs to be modified outside of rez or rolled back to a previous version until a valid one is found.

Rez-gui should probably throw the error but allow the rxt file to open so the error can be fixed within rez-gui.

Start by reproducing the failure in rez-gui with an .rxt file that references a package missing from disk. Trace the context-opening path and determine how the missing-package error is handled; done means the file opens in rez-gui, the error remains visible, and the invalid reference can be corrected there.

Written by the indexing model from the issue text.