Unclean error handling

[Peter Šurda]

Should check for missing value.

CRITICAL - File "/home/pi/PyBitmessage/src/bitmessageqt/foldertree.py", line 189, in setData
    shared.config.set(str(self.address), 'label', str(value.toString()))
File "/usr/lib/python2.7/ConfigParser.py", line 753, in set
    ConfigParser.set(self, section, option, value)
File "/usr/lib/python2.7/ConfigParser.py", line 396, in set
    raise NoSectionError(section)
CRITICAL - <class 'ConfigParser.NoSectionError'>: No section: 'None'

[Peter Šurda]

Probably fixed by dfe66ae