[r=elachuni] skip ahead to a number that is likely available when generating a sequence number to append to a duplicate username

skip ahead to a number that is likely available when generating a sequence number to append to a duplicate username

When getting an available username, add a check to reuse the existing username if the user already has one and the openid nickname matches it

Cleanup exception class heirarchy and imports, make overriding the login failure handler a settings option, add additional test cases to veryfiy that overriding the handler works and that the proper exceptions are being passed

RED: Long first/last names must be limited to 30 chars (and some refactoring of tests for readability)

Let exceptions bubble up and pass them on to render_failure so we can give users helpful information when their login fails