Improve validity check of email addresses nus-cs2103-AY2122S2#71 (nus…

…-cs2103-AY2122S2#78)

The email validation regex does not detect certain incorrect email
formats and wrongly rejects correct ones.

Let's
* improve the regex to better account for email formats  encountered in
  real life.
* modify unit tests to test the new regex.
* refine the messages shown to users to better describe the correct format
  for emails.

src/main/java/seedu/address/model/person/Email.java

@@ -45,7 +45,7 @@ public Email(String email) {
     }
 
     /**
-     * Returns if a given string is a valid email.\
+     * Returns if a given string is a valid email.
      */
     public static boolean isValidEmail(String test) {
         return test.matches(VALIDATION_REGEX);