Report java failures from the Node.js wrapper instead of returning empty output

[marevol]

Stacked on #18. Base branch is fix/nodejs-jar-selection; review that one
first. #18 fixes which jar the wrapper runs - this one fixes why running the
wrong one looked like success.

The defect

Three ways the wrapper could not report a failure.

1. Only the string [ERROR] counted as an error.

child.on('exit', function() {
    var err = null;
    if (errBuffer.indexOf('[ERROR]') > -1) { err = errBuffer; }
    callback(err, buffer, errBuffer);
});

Anything that killed the JVM before the compressor could print that marker came
back as err === null with '' for the compressed output. That is how the
NoClassDefFoundError in #18 became a silently empty minified file rather than
a build failure. The exit code was right there in the handler and unused.

2. java missing from PATH never called back at all.

spawn emits 'error' and never emits 'exit', and nothing listened for it,
so the ENOENT surfaced as an uncaught exception in whatever code happened to be
running - and the caller's callback was never invoked.

3. A read error was discarded.

getString(str, function(err, str, options) {
    compressString(str, options, callback);   // err dropped
}, options);

A path that exists but cannot be read (a directory, a permissions failure) left
str undefined, and child.stdin.write(undefined) threw
ERR_INVALID_ARG_TYPE from inside the wrapper.

Change

• A non-zero exit is an error whatever stderr contains.
• 'error' on the child, and a broken pipe on its stdin, are both handled; a
  single settle() guarantees the caller is called back exactly once.
• The read error is passed to the caller.

The existing contract is preserved: when the compressor itself reports a
problem, err is still the stderr text. Only the cases that previously reported
nothing produce an Error object, and only when stderr is empty.

Verification

The three new tests fail on the unfixed wrapper, each for its own reason:

not ok 5 - reports a non-zero exit even when stderr carries no [ERROR] marker
  error: 'a java process that exits non-zero must be reported as an error'
not ok 6 - reports a spawn failure instead of hanging
  error: 'spawn java ENOENT'   code: 'ENOENT'      <- uncaught, crashed the runner
not ok 7 - reports a file that cannot be read
  error: 'The "chunk" argument must be of type string ... Received undefined'

and pass after the change (# pass 7 # fail 0). Replaying the original
incident - wrapper pointed at the pre-shade jar - now returns the real cause
instead of an empty string:

before:  err: null                                              out: ""
after :  err: NoClassDefFoundError: org/kohsuke/args4j/...      out: ""

nodejs/cli.js is unaffected; the Java suite is unaffected (819 tests, 0 failures).