Improve error message in case invalid env var found

If you set an env var with non-yaml content but accidentally collides with a possible configuration env var,...

The current error is

```configuration error: error parsing /etc/docker/registry/config.yml: yaml: unmarshal errors:
  line 1: cannot unmarshal !!str `tcp://1...` into configuration.Parameters```

With this change we can see at least which is the problematic env var.

Some orchestrators such as docker-compose set env vars on top on user env vars, so debugging can be tricky if you are not passing vars, and the error is pointing you to a problably valid config file.

Signed-off-by: Rober Morales-Chaparro <rober@rstor.io>
Signed-off-by: Rober Morales-Chaparro <rober.morales@ebury.com>

configuration/parser.go

@@ -206,6 +206,7 @@ func (p *Parser) overwriteStruct(v reflect.Value, fullpath string, path []string
 		fieldVal := reflect.New(sf.Type)
 		err := yaml.Unmarshal([]byte(payload), fieldVal.Interface())
 		if err != nil {
+			logrus.Warnf("Error parsing environment variable %s", fullpath)
 			return err
 		}
 		field.Set(reflect.Indirect(fieldVal))