Improve "Input data was truncated" error message

[AsafMah]

Consider:

msgspec.json.decode(b"[[1,2,3,4,5], [3,4,5,6,7], [8,")

Will throw:

DecodeError: Input data was truncated

Which is pretty lacking in information.

By the point its throwing it (correct me if i'm wrong), we've already parsed some of the data.

It would be useful to know:

1. What object was failed to parse? what was its type, and where it began? (in the example: the location of the [ before the 8, and then the first [)
2. What are the objects that have been parsed so far? (in the example: [1,2,3,4,5], [3,4,5,6,7], and 8)

The scenario I'm looking for is implementing streaming-like parsing for a very specific case, and I know streaming isn't planned (#27), so I'm trying to find other small features that might be possible and will help me.

In this case, resuming to parse after more data arrives, without re-parsing everything.

[jcrist]

We can definitely add more context to the truncated error message to help with debugging. We won't ever provide a way to access half-parsed messages though.

If possible, can you open a new issue with your exact real problem? It's easier for me as a maintainer to suggest solutions to the actual issue you're dealing with, not workaround ideas you've come up with.

[AsafMah]

The scenario is to use it to parse data that might still be incomplete, and is still coming from the server. Since that's a non-goal, I'm closing.