Reflect minification result using the exit code

[garside]

When the underlying minifier runs, it "silently fails" as Juicer returns with a 0 exit code regardless of the status of the underlying minifier.

If the tool could bubble up any underlying compiler exceptions through a non-zero exit code, these sort of compiler failures can be programmatically detected and dealt with.

Example:

~/ $ juicer merge lib/jquery/jquery/jquery.js --skip-verification
[ERROR] 4051:32:missing name after . operator

... snip ...

[ERROR] 1:0:Compilation produced 30 syntax errors.
org.mozilla.javascript.EvaluatorException: Compilation produced 30 syntax errors.
... snip ...
at com.yahoo.platform.yui.compressor.Bootstrap.main(Bootstrap.java:21)
Produced lib/jquery/jquery/jquery.min.js from
lib/jquery/jquery/jquery.js

~/ $ echo $?
0