Fixes issue 439: Added exitCode enum to differentiate between different error conditions

Issue link: https://github.com/Microsoft/TypeScript/issues/439

As per the suggestions in the issue, added an enum to indicate the different exitCodes - (success, parseError, semanticError, emitError, commandLineError). Updated the corresponding code in tsc.ts to utilize the enum.
Do we want to call this DeclarationEmitError? There're no other kind of emit errors.