error strings: Don't capitalize, use periods, or newlines

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

1 files changed, 1 insertions, 1 deletions

diff --git a/prune.go b/prune.go
index cc5907a..8e35b99 100644
--- a/prune.go
+++ b/prune.go
@@ -17,7 +17,7 @@ type PruneOptions struct {
 	Handler PruneHandler
 }
 
-var ErrLooseObjectsNotSupported = errors.New("Loose objects not supported")
+var ErrLooseObjectsNotSupported = errors.New("loose objects not supported")
 
 // DeleteObject deletes an object from a repository.
 // The type conveniently matches PruneHandler.