trap inconsistency between platforms

[Thorsten Kampe <thorsten@thorstenkampe.de>]

Hi,

the below code fragment generates...

TRAPPED ON ERROR
TRAPPED ON EXIT

...on Cygwin, Linux and macOS with Bash and on Cygwin 
and macOS with Zsh.

On Linux with Zsh it only generates...

TRAPPED ON ERROR

```
# will also run on error (except with zsh on Linux)
exit_handler() { echo "TRAPPED ON EXIT"; }

error_handler() {
    error_code=$?
    echo "TRAPPED ON ERROR"
    exit $error_code
}

trap exit_handler EXIT
trap error_handler ERR

false
```

Cygwin 2.8.0, zsh 5.3.1, bash 4.4.12
Ubuntu 17.04, zsh 5.2, bash 4.4.5
macOS Sierra, zsh 5.3.1, bash 4.4.12

Thorsten