Bug fixes and UI touch-ups to interpreter config.

-Fix critical bug in AutoConfigMaker#autoConfigSingleApply.
-Adjust the way errors are displayed for auto-config failures.
-Display the name of the failed interpreter in error dialogs/messages
for all configuration failure cases.
-In AutoConfigMaker, use a new shell every time a shell is needed
rather than the same one all the time, to prevent possible problems
that might occur if the dialog a background config operation was
started in is closed.