resources: Improve getImageOps error message

Closes #14571

resources/resources_integration_test.go

@@ -97,7 +97,7 @@ Width: {{ $svg.Width }}
 		}).BuildE()
 
 	b.Assert(err, qt.IsNotNil)
-	b.Assert(err.Error(), qt.Contains, `error calling Width: this method is only available for raster images. To determine if an image is SVG, you can do {{ if eq .MediaType.SubType "svg" }}{{ end }}`)
+	b.Assert(err.Error(), qt.Contains, `error calling Width: resource "/circle.svg" of media type "image/svg+xml" does not support this method: use reflect.IsImageResource, reflect.IsImageResourceProcessable, or reflect.IsImageResourceWithMeta to check if the resource supports this method before calling it`)
 }
 
 // Issue 10255.

resources/transform.go

@@ -395,12 +395,20 @@ func (r resourceAdapter) WithResourceMeta(mp resource.ResourceMetaProvider) reso
 func (r *resourceAdapter) getImageOps() images.ImageResourceOps {
 	img, ok := r.target.(images.ImageResourceOps)
 	if !ok {
-		if r.MediaType().SubType == "svg" {
-			panic("this method is only available for raster images. To determine if an image is SVG, you can do {{ if eq .MediaType.SubType \"svg\" }}{{ end }}")
-		}
-		panic("this method is only available for image resources")
+		instructions := "use reflect.IsImageResource, " +
+			"reflect.IsImageResourceProcessable, or " +
+			"reflect.IsImageResourceWithMeta to check if the resource " +
+			"supports this method before calling it"
+		msg := fmt.Sprintf(
+			"resource %q of media type %q does not support this method: %s",
+			r.Name(),
+			r.MediaType(),
+			instructions,
+		)
+		panic(msg)
 	}
 	r.init(false, false)
+
 	return img
 }