When I invoke Z-80 assembler on a program containing non-existent
instrution "cp (hl),N", the compilation goes without any error (or at
least warning) messages. It seems, that even some object code is
generated for this instruction. Here is transcript:

$ echo 'cp (hl),239' > bad.asm
$ z80asm -v bad.asm -o bad.bin
Assembling....
Assembly succesful.
$ echo $?
0
$ xxd bad.bin
0000000: be                                       .
$

Note: 'be' is the opcode for 'cp (hl)', so an error message "trailing
characters ',239'" seems to be appropriate.