Print an error level message if package does not start with a letter

[nbbrooks]

While doing some adversarial testing, we found that we could attempt to create a ROS 2 C++ package that starts with a letter and ament would print a warning level message (I believe using catkin's package naming checks). There would be subsequent compilation errors of course, but it would be good to flag this as an Error earlier if possible.

WARNING: Package name "1test" does not follow the naming conventions. It should start with a lower case letter and only contain lower case letters, digits, underscores, and dashes.

Is there a reason the error log level check allows for starting with a number?

If we agree on this change I will update existing unit tests as needed.