fix: recover from panic in caching of invalid jsonnet

In some cases, requesting a list of dependencies on an invalid jsonnet file will lead to a panic deep in the jsonnet interpreter.

This leads to a very poor developer experience for end-users, as the user gets a random stacktrace from Go/jsonnet-tool, rather than a message about the invalid jsonnet file.

This improves jsonnet-tool to better improve the usability of this situation.