Fix: Hiredis parser traps pubsub event exceptions

[felixge]

While using this module with the hiredis parser, I noticed that exceptions
thrown inside of subscribe/message/etc. callbacks would be reported as
parser error exceptions, which was confusing.

Example:

var client = require('./index').createClient();

client.on('subscribe', function() {
  throw new Error('My Error');
});
client.subscribe('channel');

I would expect the above to throw 'My Error' as a simple exception, however,
when using the hiredis parser I get:

events.js:45
        throw arguments[1]; // Unhandled 'error' event
                       ^
Error: Redis reply parser error: Error: My Error
    at RedisClient.<anonymous> (/Users/felix/code/node_redis/hiredis.js:4:9)
    at RedisClient.emit (events.js:67:17)
    at RedisClient.return_reply (/Users/felix/code/node_redis/index.js:457:22)
    at HiredisReplyParser.<anonymous> (/Users/felix/code/node_redis/index.js:220:14)
    at HiredisReplyParser.emit (events.js:64:17)
    at HiredisReplyParser.execute (/Users/felix/code/node_redis/lib/parser/hiredis.js:35:22)
    at RedisClient.on_data (/Users/felix/code/node_redis/index.js:365:27)
    at Socket.<anonymous> (/Users/felix/code/node_redis/index.js:58:14)
    ...

The attached patch fixes this problem by changing the way the HiredisReplyParser
traps exceptions, and also provides a unit test for it.

Please let me know if you need any tweaks on the patch for landing it, I was
not quite sure where to put my test / how to structure it, since most test
cases seem to be integration tests at this point.

Also, if you would prefer an integration test for this, here you go:

var client = require('./index').createClient();
var assert = require('assert');

process.on('uncaughtException', function listener(err) {
  process.removeListener('uncaughtException', listener);

  // Drop the stack, otherwise the assert gets trapped by node
  process.nextTick(function() {
    assert.ok(!/parser error/.test(err.message), err.message);
    client.quit();
  });
});

client.on('subscribe', function() {
  throw new Error('My Error');
});
client.subscribe('channel');

PS: If you wonder why this only affects pubsub events: This is because
RedisClient#return_reply does exception trapping differently for events than
it does for callbacks.

See: https://github.com/mranney/node_redis/blob/3e95c55a0301104c9cd6451d71fa5527b7797792/index.js#L431
Vs: https://github.com/mranney/node_redis/blob/3e95c55a0301104c9cd6451d71fa5527b7797792/index.js#L447

[felixge]

Just noticed that I used 2 spaces instead of 4 in some places, I'll fix that if you confirm the overall patch to be good.

[rmehner]

+1. Makes it way easier to debug applications that use node-redis.

[mranney]

This is a win, thanks. This will go into the next version that I'm working on now.