Exit 94 if a mirror lock times out

[DrJosh9000]

Description

Lockfiles (used for serialising access to git mirrors) aren't totally reliable in the face of agents dying without releasing them. As a stopgap, if a lockfile can't be acquired before the timeout, exit with a distinctive code (94, chosen randomly).

Context

https://coda.io/d/_dHnUHNps1YO#Escalations-Pipelines_tuQbCBf6/r711

Changes

• Create a new error type, ErrTimedOutAcquiringLock.
• Return it if the git mirror lockfiles fail for context.DeadlineExceeded
• Check for it in the checkout phase, and wrap it in a shell.ExitError
• Update the one other use of shell.ExitError

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)

[wolfeidau]

Great work, should we add something to the docs for agent error codes?

[DrJosh9000]

Great work, should we add something to the docs for agent error codes?

Yes we should!