Issue reported by elunna: the message given when zap_over_floor()
hits iron bars with lightning or acid was substituting a couple of
words or phrases in the wrong order, resulting in
|The {melt|dissolve} iron bars somewhat but remain intact.
when the iron bar location is flagged as non-diggable. It should be
|The iron bars {melt|dissolve} somewhat but remain intact.
Not mentioned: the corresponding message for locations that aren't
flagged as non-diggable used "melt" unconditionally. Change it to
keep "melt" for lightning but switch to "corrode away" for acid.
Fixes#1342
1e431139b4