Display a better error message when parsing protobuf file descriptor sets fails

[foxymiles]

Description
The error we display when we fail to parse a protobuf file descriptor set is not very helpful.

• Version: 1.4.0
• OS: macOS

Steps To Reproduce

1. Load an mcap file with invalid protobuf schema (see bad_protobuf_schema.mcap in the corrupt folder).

Current Experience
The error message we display here doesn't help the user understand the problem.

Desired Experience
Provide a more useful and actionable error message.