rendertestsuite: don't give up on the first unloadable file

Some files in the SVG 1.2 Tiny suite are meant to fail to load:
struct-use-recursion-01-t.svg, for instance, is a deliberate cycle that
the parser discards. render() returned a null QImage for those and main()
turned that into EXIT_FAILURE, so create-baseline abandoned the run at
the first one and left a partial baseline behind.

Render such files blank and carry on, the way the baseline test does, and
skip rather than fail in diff mode when a reference image is missing, so
a partial baseline from an earlier run can still be compared.

Change-Id: Iba143bfd0162eed974384ccd8d85e6498e8d2142
Reviewed-by: Hatem ElKharashy <hatem.elkharashy@qt.io>