Suppress indent errors during electric indentation (Bug#18764)

Suppress indent errors during electric indentation (Bug#18764)

* lisp/electric.el (electric-indent-post-self-insert-function):
Suppress errors from indent code, but don't suppress errors from
elsewhere in this function.  That way, if trouble is encountered with
electric indent "not working", the error should be reproducible by
calling indent directly (as is the case for Bug#18764), or else it's
from the electric indent code and will be reported normally.