GitLab

First letter of an error string should not be capitalized, as prescribed in the [wiki](https://github.com/golang/go/wiki/Errors).

Change-Id: Iea1413f19b5240d3bef79f216094d210b54bdb62
GitHub-Last-Rev: d8e167107122b603c4f647d722537668ad1c680d
GitHub-Pull-Request: golang/go#35203
Reviewed-on: https://go-review.googlesource.com/c/go/+/203797

Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>