refactor: Remove old error handling library

Removes the old error handling library and switches to plain
fmt.Errorf calls.

There are several reasons for this:

* There are no useful types or handling here anyways, so output format
  is the only priority.
* Users don't care about getting stacktraces.
* My emotional wellbeing.

Fin de siècle.

example/some-api/some-api.yaml

@@ -4,7 +4,7 @@ kind: Secret
 metadata:
   name: secret-certificate
 data:
-  cert.pem: { passLookup "my/secret/certificate" | b64enc }}
+  cert.pem: {{ passLookup "my/secret/certificate" | b64enc }}
 ---
 apiVersion: extensions/v1beta1
 kind: ConfigMap