[PATCH] pglister: auth_receive: Indicate when PGAUTH_KEY is invalid instead of crashing

When Django is improperly configured with a PGAUTH_KEY that does not match the one configured in pgweb, the decryptor
returnsa string that can't be utf-8-decoded. Catching this error makes it possible to return a proper error message
insteadof a 500 error.
 
Of course, the unicode error can be due to other issues (e.g. tempered GET parameters), but wrong PGAUTH_KEY is the
morelikely issue to happen when configuring pglister.
 

Patch attached.
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Célestin Matte