Use strings for error codes

[alecgibson]

We should use Node-style error codes to be consistent with sharedb.

[ericyhwang]

Error codes for sharedb-mongo:

sharedb-mongo/index.js

Lines 1551 to 1618 in 2287e66

	// Bad request errors
	ShareDbMongo.invalidOpVersionError = function(collectionName, id, v) {
	return {
	code: 4101,
	message: 'Invalid op version ' + collectionName + '.' + id + ' ' + v
	};
	};
	ShareDbMongo.invalidCollectionError = function(collectionName) {
	return {code: 4102, message: 'Invalid collection name ' + collectionName};
	};
	ShareDbMongo.$whereDisabledError = function() {
	return {code: 4103, message: '$where queries disabled'};
	};
	ShareDbMongo.$mapReduceDisabledError = function() {
	return {code: 4104, message: '$mapReduce queries disabled'};
	};
	ShareDbMongo.$aggregateDisabledError = function() {
	return {code: 4105, message: '$aggregate queries disabled'};
	};
	ShareDbMongo.$queryDeprecatedError = function() {
	return {code: 4106, message: '$query property deprecated in queries'};
	};
	ShareDbMongo.malformedQueryOperatorError = function(operator) {
	return {code: 4107, message: 'Malformed query operator: ' + operator};
	};
	ShareDbMongo.onlyOneCollectionOperationError = function(operation1, operation2) {
	return {
	code: 4108,
	message: 'Only one collection operation allowed. ' +
	'Found ' + operation1 + ' and ' + operation2
	};
	};
	ShareDbMongo.onlyOneCursorOperationError = function(operation1, operation2) {
	return {
	code: 4109,
	message: 'Only one cursor operation allowed. ' +
	'Found ' + operation1 + ' and ' + operation2
	};
	};
	ShareDbMongo.cursorAndCollectionMethodError = function(collectionOperation) {
	return {
	code: 4110,
	message: 'Cursor methods can\'t run after collection method ' +
	collectionOperation
	};
	};
	// Internal errors
	ShareDbMongo.alreadyClosedError = function() {
	return {code: 5101, message: 'Already closed'};
	};
	ShareDbMongo.missingLastOperationError = function(collectionName, id) {
	return {
	code: 5102,
	message: 'Snapshot missing last operation field "_o" ' + collectionName + '.' + id
	};
	};
	ShareDbMongo.missingOpsError = function(collectionName, id, from) {
	return {
	code: 5103,
	message: 'Missing ops from requested version ' + collectionName + '.' + id + ' ' + from
	};
	};
	// Modifies 'err' argument
	ShareDbMongo.parseQueryError = function(err) {
	err.code = 5104;
	return err;
	};