Add file checks to deploy step

[dkundel]

As pointed out in #465 we currently don't do any specific validation of the contents of directories for functions and assets and instead put that validation on the client which results in cryptic error messages. Instead we should add an additional validation step inside twilio-run that checks the contents of the functions and assets directory and makes sure that there are at least one Function or Asset found. If none can be found we should output an error specifying that no Functions or Assets were found and output the locations where the tool looked.