Improve error messaging

Currently, the program ignores values that don't meet the following criteria:

* The value must be either 3 or 6 characters long (not in-between)
* The value must be a hexadecimal digit (i.e. 0-9, and A-F, where letter values can be either upper or lower-case)

However, we want to give users specific error messages associated with the corresponding values, to see _why_ values are excluded:

- [ ] When a value's length is invalid, print to the console: `___ ERR_LENGTH: RGB values must be either 3 or 6 characters long` where `___` is replaced with that particular value
- [ ] When a value contains one or more characters that is not a hexademical digit, print to the console: `___ ERR_VALUE: RGB values must be in 0-F range` where `___` is replaced with that particular value

In addition, if no values are provided (i.e. `argc` is below 2), then before the program quits, it should print to the console: `ERR_MISSING: One or more RGB values should be provided as arguments, separated by spaces`