setConfig should not set invalid key

[dariober]

setConfig can set an invalid key thus making ASCIIGenome stuck in error state until the user re-sets the offending key to a valid one. E.g.:

ASCIIGenome -r chr7:5529435-5529716 test_data/ear045.oxBS.actb.bam

[h] for help:  setConfig shade_structural_variant FOO 
Unrecognized color: foo
Unable to set configuration   

[h] for help:  setConfig -h
...
     shade_structural_variant           FOO   # Background... <- This should not have changed! 
...

[h] for help:  zo
Unrecognized color: foo
Error processing tracks with input [zo]  

[h] for help:  setConfig shade_structural_variant 10 // <- Now back to normal

Expected behaviour: setConfig shade_structural_variant FOO throws the error AND keeps the current value of the key.

ASCIIGenome 1.16.0