Tweak import paths of .proto files in errorspb

[rickystewart]

This makes imports consistent both within this repository as well as in
the main CockroachDB repo. This simplifies the Bazel migration as seen
in cockroachdb/cockroach#57974.

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This change is 

[CLAassistant]

All committers have signed the CLA.

[knz]

LGTM thanks

[tooolbox]

Out of curiosity, is this change backwards compatible, i.e. markers encoded with the old registered name will properly decode with the new registered name, and vice versa?

[rickystewart]

To my knowledge, yes (and this change has been live at CockroachDB master for more than 2 weeks, so I imagine that any issues would have been noticed by now).

[knz]

markers encoded with the old registered name will properly decode with the new registered name, and vice versa?

Thanks for noticing. I agree we need to explicitly check this.

I imagine that any issues would have been noticed by now).

No not necessarily. Last time we broke something in that area, we only noticed when a customer encountered a production outage after an upgrade 😭

[knz]

@rickystewart also you don't need to do anything about this; I will do the investigation and fix the fallout if any.