"lei import" should never be without a {sto}, and *_done should
not be called multiple times, so ensure we can fail if it's
missing.
Update some existing tests to complain loudly by introducing a
handy "xbail" function which wraps "explain" and BAIL_OUT.
BAIL_OUT was painful to type and concatenating the result of
"explain" doesn't work as I thought it would since "explain"
always returns an array, and BAIL_OUT only accepts a single
scalar arg (unlike "die").