Per [Go Code Review Comments][1],
> Error strings should not be capitalized (unless beginning with proper
> nouns or acronyms) or end with punctuation
staticcheck's [ST1005][2] also complains about these. For example,
```
object_walker.go:63:10: error strings should not be capitalized (ST1005)
object_walker.go:101:10: error strings should not be capitalized (ST1005)
object_walker.go:101:10: error strings should not end with punctuation or a newline (ST1005)
plumbing/format/commitgraph/file.go:17:26: error strings should not be capitalized (ST1005)
```
This fixes all instances of this issue reported by staticcheck.
[1]: https://github.com/golang/go/wiki/CodeReviewComments#error-strings
[2]: https://staticcheck.io/docs/checks/#ST1005
