Migrate error comparisons to errors.Is/errors.As

[bobheadxi]

This change migrates direct error comparisons and error type assertions to the
modern errors package idioms:

• err == SentinelErr / err != SentinelErr -> errors.Is(err, SentinelErr) / !errors.Is(...)
  for sentinels such as sql.ErrNoRows, context.Canceled, context.DeadlineExceeded,
  http.ErrServerClosed, io.ErrUnexpectedEOF, os.ErrNotExist, etc.
• Error type assertions and switch err.(type) classification -> errors.As.

Excluded by design: io.EOF comparisons (idiomatic in reader loops) and _test.go files.

This makes error checks robust to wrapped errors (fmt.Errorf("...: %w", err)).

Generated by a Sourcegraph batch change.

Created by Sourcegraph batch change robert/58006530-3ddc-4c11-ad42-8d8eed8bc63f.