server: show user feedback when key is anonymous

When an ollama key is not registered with any account on ollama.com this is
not obvious. In the current CLI an error message that the user is not
authorized is displayed. This change brings back previous behavior to show
the user their key and where they should add it. It protects against adding
unexpected keys by checking that the key is available locally.

A follow-up change should add structured errors from the API. This change
just relies on a known error message.

types/errtypes/errtypes.go

@@ -16,6 +16,6 @@ type UnknownOllamaKey struct {
 	Key string
 }
 
-func (e *UnknownOllamaKey) Error() string {
+func (e UnknownOllamaKey) Error() string {
 	return fmt.Sprintf("unauthorized: %s %q", UnknownOllamaKeyErrMsg, strings.TrimSpace(e.Key))
 }