Add more validations of user input and instructions to cd <app-dir>

/to @bajtos 
/cc @altsang 
/cc @cgole 

The PR includes the following fixes:
- adds a conditional message if the app dir is not `.`.
- adds more validations to user input for names
- fixes the url-not-found warnings on jquery map and favicon for loopback:example

The error message must tell the user how to fix the name to become valid. Either list all rules in the message, or tell him which rule was violated.