[BUG] Postprocessor failure is not detected

[hdwalters]

Describe the bug
If a postprocessor returns a non-zero (error) exit code, it is not detected, and the empty standard output is returned instead.

To Reproduce
To reproduce this, create an Amber script with an unterminated if statement inside $...$ delimiters:

hwalters@Ghostwheel ~/git/amber-lang (master) 
$ cat failing.ab 
trust $ if $

We can confirm that shfmt rejects this invalid Bash as follows:

hwalters@Ghostwheel ~/git/amber-lang (master) 
$ shfmt <<EOF
> if
> EOF
<standard input>:1:1: "if <cond>" must be followed by "then"

However, when the Amber script is compiled to Bash, the postprocessor ignores the shfmt error:

hwalters@Ghostwheel ~/git/amber-lang (master) 
$ amber failing.ab -
#!/usr/bin/env bash
# Written in [Amber](https://amber-lang.com/)
# version: 0.3.5-alpha
# date: 2024-11-09 22:14:43

Expected behavior
Amber should show the contents of the standard error stream returned by the postprocessor:

hwalters@Ghostwheel ~/git/amber-lang (master) 
$ amber failing.ab -
 ERROR  Postprocessor 'shfmt' failed
<standard input>:1:1: "if <cond>" must be followed by "then"

Additional context
This is happening because the ExitStatus (returned by the Child subprocess in the postprocessor code) is not checked for success. It will also be necessary to propagate the error result back up the call chain.