Gnus error checking in encode-coding-*

Is there a standard unified way to do error checking in encode-coding-*?
As an example, I get

> (encode-coding-string "ä" 'us-ascii)
"?"

with no obvious way to ask `encode-coding-string' to raise an error. I
suppose, I could just convert the result ("?" in this case) back into
internal representation and check it against the original string, but is
this really the right strategy? It would seem that something like this
should be automated.

This question is specifically geared towards Gnus. Maybe it's just not
in the Gnus tradition or philosophy?

On the other hand, the ELisp library is not Gnus-specific. So, what's
the general philosophy here?


Many thanks.