metomi/rose#597: improve colour parse failing
metomi/rose#597: changes following review
rose.popen: ensure OSError has a filename

OSError is normally raised if a command cannot be executed.
t/rose.popen: connected
Merge pull request #901 from benfitzpatrick/597.colour-spec

#597: improve colour parse failing
Merge pull request #912 from matthewrmshin/rose.popen.oserror

rose.popen: ensure OSError has a filename
Merge branch 'master' into rose-bush