Awkward/incorrect English wording "has existed"

[movermeyer]

• Operating system: All
• Pencil version: 3.0.2

The wording of two dialog boxes is not proper English. This was originally noticed by @thinw002, as reported in #320:

• The page name 'Test1' has existed should be The page name 'Test1' already exists
  similarly:
• Font name 'TestFont' has existed. should be Font name 'TestFont' already exists.

A pull request will arrive shortly.

[movermeyer]

You might want to consider using some internationalization (i18n) library to allow for easier translation of Pencil into other languages.