New Submission Form - Improve plugin upload error messages

[Ipstenu]

Currently if you try to upload a plugin that already exists, you get this:

There is already a plugin called hello-dolly by a different author. Please change the name of your plugin and upload it again.

Since we *can* rename plugins if not-approved (yay! See #1686) we should have a better error that explains where the name is being pulled from here.

There is already a plugin called hello-dolly by a different author. Please change the name of your plugin in the plugin header file and upload it again.

I've updated the plugin add page to have a FAQ that (I hope) will explain this better...

[ChopinBach]

Changes the error message for the Plugin Upload Handler for plugin names that already exit.

[obenland]

In 3447:

Plugin Directory: More helpful error message on duplicate plugin name.

Props ChopinBach.
Fixes #1700.

[samuelsidler]

Milestone Plugin Directory v3 - M5 deleted