• [*] Your changes are not possible to do through a plugin and relevant
  to a large audience (ideally all users of OctoPrint)
• [N/A] If your changes are large or otherwise disruptive: You have
  made sure your changes don't interfere with current development by
  talking it through with the maintainers, e.g. through a
  Brainstorming ticket
• [*] Your PR targets OctoPrint's devel branch if it's a completely
  new feature, or maintenance if it's a bug fix or improvement of
  existing functionality for the current stable version (no PRs
  against master or anything else please)
• [*] Your PR was opened from a custom branch on your repository
  (no PRs from your version of master, maintenance or devel please),
  e.g. dev/my_new_feature or fix/my_bugfix
• [*] Your PR only contains relevant changes: no unrelated files,
  no dead code, ideally only one commit - rebase and squash your PR
  if necessary!
• [*] Your changes follow the existing coding style
• [*] If your changes include style sheets: You have modified the
  .less source files, not the .css files (those are generated with
  lessc)
• [*] You have tested your changes (please state how!) - ideally you
  have added unit tests
• [*] You have run the existing unit tests against your changes and
  nothing broke
• [*] You have added yourself to the AUTHORS.md file :)

What does this PR do and why is it necessary?

• Changing login error message from 'Incorrect username or password.' to 'Incorrect username or password. Hint: Both are case sensitive!'

How was it tested? How can it be tested by the reviewer?

• Run octoprint
• Enter incorrect credentials
• Verify 'Incorrect username or password. Hint: Both are case sensitive!' message is shown
• Enter correct credentials login is successful

Any background context you want to provide?

N/A

What are the relevant tickets if any?

#4227

Screenshots (if appropriate)

Further notes

N/A