Improve error message upon failed copy(file, dir)

[bseib]

• Operating System: Win10
• Node.js version: v10.16.0
• fs-extra version: 8.1.0

When you attempt to call copy('./some-file', './some-dir') you will get an error message that does not reflect what the docs are telling me not to do. The error message you get is about unlink, and I spent too much time tracking down what was really going on in a recent bit of code.

I now understand from issue #323 that it is intentional that a person should not try to call copy() with a file as the source and a directory as the destination, and I support the reasoning to have people be explicit in their intent. When I read the documentation for copy(), I see that this is explained clearly. But I wasn't the original author of the code and I wasn't the one reading the docs before writing the code. I was the debugger of code that came before me.

I propose that an error message be given that is consistent with the copy() documentation when the first arg is a file and the second arg is a directory. If that combination of arguments is documented to never be allowed, then the error message should reflect that.

[RyanZim]

@manidlou Seems doable, what do you think?

[manidlou]

Yeah I agree! In fact, we have implemented the proper message for the reverse case when src is a directory like copy('./some-dir', './some-file').

https://github.com/jprichardson/node-fs-extra/blob/master/lib/copy/copy.js#L146

Maybe it is better we handle that for both cases when we check the src and dest stats at the beginning.

[RyanZim]

Fixed in #779