a nicer error message when using %pylab magic

If the user specified an unsupported backend, there was no indication
for what the valid names were. Error message prior to this commit:

    In[1]: %pylab xxx
    ERROR: Backend u'xxx' not supported.

after this commit:

    In[1]: %pylab xxx
    ERROR: Backend 'xxx' not supported. Supported backends are: gtk inline osx qt qt4 tk wx
Merge pull request #3608 from ivanov/nicer-backend-msg

a nicer error message when using %pylab magic

If the user specified an unsupported backend, there was no indication
for what the valid names were. Error message prior to this commit:

In[1]: %pylab xxx
ERROR: Backend u'xxx' not supported.
after this commit:

In[1]: %pylab xxx
ERROR: Backend 'xxx' not supported. Supported backends are: gtk inline osx qt qt4 tk wx