Added Enhancement: username-taken suggestion feature for registering new user

[LeyouJcmu]

Currently, when users register with a taken username, it displays "Username taken."

This commit changes the message to include an alternative username (of the form) "${username}suffix", where ${username} is the username they are attempting to use in registration.

The primary changes to the code were to:
public/src/client/register.js - to pass in the attempted username into the username-taken error
public/language/en-US/error.json - to display the new username-taken message

This PR closes #1

Example of fix:

[coveralls]

Pull Request Test Coverage Report for Build 21594911267

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.841%

---

Totals
Change from base Build 21503398961:	0.0%
Covered Lines:	25358
Relevant Lines:	30328

---

💛 - Coveralls