Improve the error when imports are written before a part of directive

[DanTup]

With enhanced parts, it seems imports should be before part directives, but they come after part of directives. This makes some sense, but it's easy to get wrong. If I put an import after a part, I get a clear error message:

However the same is not true if I get a part of and import the wrong way around:

The error makes it seem like I can't have imports in this file, but in reality I just need to put them after the part of.