lib: If i_stream_default_seek_nonseekable() can't seek, set a more readable error message.
lib: If i_stream_default_seek_nonseekable() can't seek, set a more readable error message.

LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot