Throwing between in GC programs is annoying

[p0nce]

Simplify these patterns:

if (!img.isValid)
    throw new Exception(img.errorMessage().idup);

Also this pattern is wrong! No error message on saving files.

if (!image.saveToFile("path.ext"))
    throw new Exception(img.errorMessage().idup);

• Should failed save should put the image in errored state?
• Helper function for first pattern
• Decide what to do for last pattern