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/shhg initecho a > ahg ci -Am thg mv a bhg ci -Am t1hg debugrename bhg mv b ahg ci -Am t2hg debugrename aecho % test with --revhg debugrename --rev 1 b