Improve error diagnostics

[cijiugechu]

Some cli failures currently lack enough context (#234 , #210). This increases debugging time for users.

Proposal

Add missing context + preserve error sources

• Wrap errors at boundaries with operation/resource context.
• Ensure file path / URL is included where we already know it.
• Keep underlying errors as sources.

Open question

• Is introducing anyhow / miette acceptable ?

cc @arcanis , if this direction makes sense to you, I’m happy to submit PRs.

[arcanis]

Gladly! The JSON error messages in particular are really hard to read. Some thoughts:

• If possible I'd prefer to keep most messages one-liners. That's for example why there's the concept of ChildProcessFailedWithLog where we only show a single line and send stdout/err into a separate file.

• Although this can be relaxed at the end of the process. For example all ChildProcessFailedWithLog entries are collected within the CI and printed at the end of the execution.

• I tried to keep most errors in error.rs because I had the idea to eventually use a custom macro that would accept long definitions and would make it easy to generate up-to-date documentation similar to this.

[cijiugechu]

As an experiment, this PR does not introduce anyhow/miette. Instead, it adds extra fields to the Error type to carry additional context. Since the additional fields increase the type size and hurt cache locality, some String fields have been switched to EcoString to mitigate the performance impact.