New function to return the current nil pattern depending on the status.
Add missing early bailout on surface->status with error propagation to the pattern.
Related cleanups for cairo_pattern_t:
Don't check other->status since this is a static function.
Add missing early bailout on other->status.
Cleanup identifier names.
Track rename of nil patterns.
Don't call _cairo_error for pre-existing errors.
Take care to initialize some fields to that _cairo_pattern_release_surface will work even after an error.
Track rename of cairo_solid_pattern_nil to cairo_pattern_nil.
New test to ensure that a file-not-found error will propagate from a surface, through a pattern, and onto a cairo_t.
