Plugin Directory: Use a consistent wording for upload error messages

[SergeyBiryukov]

• Some strings refer to the "plugin header file", others to "your main plugin file".
• "Please change the name of your plugin" might not be clear enough, it should refer to the Plugin Name: line.

[ocean90]

In 5392:

Plugin Directory: Use a consistent wording for upload error messages.

Props SergeyBiryukov.
Fixes #2757.

[ocean90]

In 5401:

Plugin Directory: Use correct placeholder.

See #2757.

[ocean90]

#2761 was marked as a duplicate.

[ocean90]

#2761 was marked as a duplicate.