Support catching @rule errors

[kaos]

Instead of returning any thrown exceptions for the execution request directly, support catching them in the rules during rule graph execution so recoverable/non critical errors may be handled.

This PR revives previous work by @cosmicexplorer in #10954 bringing it up-to-date with current main -- with some minor adjustments.

The primary difference is in the error handling; tracebacks rely more on the Exception.__cause__ attribute of the errors and rather than adding an additional layer of result wrapping for the nodes, we propagate all the Failures to the rules instead, so any exceptions thrown during rule execution is picked up as a Failure::Throw{} as before, but now it is forwarded to the parent rule to potentially catch. So, use raise .. from e in order to preserve the tracebacks of the previous error when desired.

Example:

@rule
async def catch_merge_digests_error(file_input: FileInput) -> MergedOutput:
    # Create two separate digests writing different contents to the same file path.
    input_1 = CreateDigest((FileContent(path=file_input.filename, content=b"yes"),))
    input_2 = CreateDigest((FileContent(path=file_input.filename, content=b"no"),))
    digests = await MultiGet(Get(Digest, CreateDigest, input_1), Get(Digest, CreateDigest, input_2))
    try:
        merged = await Get(Digest, MergeDigests(digests))
    except Exception as e:
        raise Exception(f"error merging digests for input {file_input}: {e}")
    return MergedOutput(merged)

[stuhood]

This looks really great! Thanks @kaos