check exec: return fallback in case of error during the check
If there is any error while checking if exec is supported,
we can return fallback.
fix issue705
#!/bin/sh
hg init dir
cd dir
echo bleh > bar
hg add bar
hg ci -m 'add bar'
hg cp bar foo
echo >> bar
hg ci -m 'cp bar foo; change bar'
hg debugrename foo
hg debugindex .hg/store/data/bar.i