Expose error code for upload failure reason

In https://github.com/filecoin-project/filecoin-pin/pull/501/changes#diff-2edc6b782492bf066084334ff910ec3612fe97810c48080752d8b04759da7b5bR85-R95, we match on error strings in order to classify the failure. It would be great if the SDK exposed error codes, that can be relied on without fearing accidental breakage when strings change.

After releasing, update Filecoin Pin to use the error codes.

Start with the linked Filecoin Pin pull request diff, especially the referenced lines that classify upload failures by error strings, then locate the corresponding error definitions and SDK release process. Done means the SDK exposes stable error codes and, after release, Filecoin Pin uses those codes instead of matching error strings.

Written by the indexing model from the issue text.