Turtleart fail gracefully on bad file

[tonyforster]

With a bad ta file, skip the bad block and load the rest of the ta file or skip the ta file but still start?

File "/home/olpc/Activities/TurtleArt.activity/TurtleArt/tablock.py", line 366, in _set_label_attributes

n = len(BLOCK_NAMES[self.name])

KeyError: u'cleanr'

TA100 373pyg XO1

[walter]

​http://git.sugarlabs.org/projects/turtleart/repos/mainline/commits/e074e2e28f6193730757830561ddd551c4b52a19

and

​http://git.sugarlabs.org/projects/turtleart/repos/mainline/commits/ddd48d34a05fb540f40fd81c61a25f99f0d7f741

should catch many more of the Value and Key errors that may be associated with bad block descriptions in TA files.

Needs testing.

[walter]

I think this version is pretty robust. Please test.