r14902: change charcnv code to fail the conversion when it hits bad

r14902: change charcnv code to fail the conversion when it hits bad
characters, rather than silently truncating the string. This makes the
code much omre conservative, making it easier to test. It might mean
users hit problems initially, but at least we'll hear about them, and
thus can fix them.
(This used to be commit bb99cbf069db5ab09ecf6c55e4c87d4c28ea8169)