finish GstMessage constructors

gst/g_error.go

@@ -1,10 +1,13 @@
 package gst
 
-// GError is a Go wrapper for a C GError. It implements the error interface
+// GError is a Go wrapper for a C GError in the context of GStreamer. It implements the error interface
 // and provides additional functions for retrieving debug strings and details.
 type GError struct {
 	errMsg, debugStr string
 	structure        *Structure
+
+	// used for message constructors
+	code int
 }
 
 // Message is an alias to `Error()`. It's for clarity when this object
@@ -19,3 +22,12 @@ func (e *GError) DebugString() string { return e.debugStr }
 
 // Structure returns the structure of the error message which may contain additional metadata.
 func (e *GError) Structure() *Structure { return e.structure }
+
+// NewGError wraps the given error inside a GError (to be used with message constructors). The code
+// is optional and allows for adding additional "types" to the error.
+func NewGError(code int, err error) *GError {
+	return &GError{
+		errMsg: err.Error(),
+		code:   code,
+	}
+}