Converting +/-50V source to 0-5V via diff amp?

I'd like to monitor a slowly changing 1V to 50V source via the ADC on
a PIC16F876A. I was originally going to use a 10:1 voltage divider
(0.1% resistors) to bring the voltage down to 0.1V-5V and apply that
to the ADC. Then I realized I would eventually accidently hook up the
source "backwards" and blow the PIC.

I was considering using a difference amp (AD628) for the job. Not
only would I not have to worry about GND problems, it would drop the
comon mode voltage that might be riding on the long leads from the
diff. amp. to the voltage source.

But, while using a 10V supply and a 2.5 Vref gives me an output of
2.6V-7.5V for a "correctly" connected source (SRC POS to +IN of amp
and SRC NEG to -IN of amp), which I would either divide-by-2 before
going to the PIC or change the amp's default 0.1 gain, what happens if
the source is connected "backwards"?

Does the amp just output zero volts (or whatever its min. voltage
swing is, IIRC, 30mV) because I'm using a unipolar supply for the amp?
Or do I end up zapping the amp?

I'd like to avoid using a bipolar supply for the amp as I don't need
the neg. voltage for anything else.

And while it would be great to be able to monitor the source no matter
how it was connected, I can't have the the PIC's ADC cover a +/-50V
source with high enough resolution. I need 10-bits available at the
DAC for a 50V range, minimum.

Adding a 12-bit ADC would do the job but I'd like to keep this as
simple as possible and I think I can if the diff. amp. setup described
above just drops to zero volts (almost) when the source is connected
"backwards". I could then use a amp output of less than 2.5V as an
indicator that the battery pack was connected backwards and sound a
klaxon, flash a strobe, send out a flare, whatever, to let me know.

Thanks for your help!
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