Improve error handling and error messaging
Extract VALID_LETTERS regex to constant for clarity
Search for invalid characters

Instead of checking that we only have valid letters in the
search string we do it the other way around.
By searching for invalid letters we can present the invalid
letters for the user in the error message.
Minor adjustments for the PR
Merge pull request #1 from karllindmark/pr10-adjustments

Minor adjustments for the PR
List ilegal letters only once

Before the ilegal letters are printed out the list is sorted and all the
duplicates are removed, so the an ilegal letter is only listed once.
Fix bug on step 3
Merge pull request #10 from wiesom/dev

Improve error handling and error messaging