ImageMagick->save() return value

[MGatner]

Description
In the Image Magick handler class, save() currently returns the result of process(), which is an array of the lines of output from exec(). This can cause a type error because the return type is defined as bool.

This PR changes save() to return true after process() calls, as process() will throw an exception on failure already and $output is unreliable for determining success or failure.

This resolves #2029.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[MGatner]

@lonnieezell This is all set unless there is some bigger picture reason to do it differently.