Lowercase some errors

This commit is courtesy of

```
for f in $(git ls-files *.go | grep -v ^vendor/); do \
	sed -i 's/\(errors\..*\)"Error /\1"error /' $f;
done

for f in $(git ls-files *.go | grep -v ^vendor/); do \
	sed -i 's/\(errors\..*\)"Failed to /\1"failed to /' $f;
done

```

etc.

Self-reviewed using `git diff --word-diff`, found no issues.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libpod/runtime.go

@@ -582,7 +582,7 @@ func (r *Runtime) Shutdown(force bool) error {
 	// attempt to shut it down
 	if r.store != nil {
 		if _, err := r.store.Shutdown(force); err != nil {
-			lastError = errors.Wrapf(err, "Error shutting down container storage")
+			lastError = errors.Wrapf(err, "error shutting down container storage")
 		}
 	}
 	if err := r.state.Close(); err != nil {