BREAKING: Do not error when copy destination exists & clobber: false

[RyanZim]

Add errorOnExist option for users that want an error

Resolves #321

@jprichardson What do you think of the error message?

[coveralls]

Coverage decreased (-0.02%) to 85.407% when pulling c09ca54 on copy-clobber into 5f319b6 on master.

[RyanZim]

Hold off on this, copySync needs edited too.

[coveralls]

Coverage decreased (-0.03%) to 85.391% when pulling 3fa83d4 on copy-clobber into 5f319b6 on master.

[RyanZim]

copySync updated.

[RyanZim]

@jprichardson I think this is merge-ready, just want you to approve the error message first.

[jprichardson]

Awesome, thanks!

[ghost]

@RyanZim

What do you think of the error message?

Just a thought - do you think throwing the same style error messages as fs returns would be better suited since this module is meant as a drop-in replacement?

Meaning that instead of ${path} already exists one might throw an EEXIST error?

[RyanZim]

@callodacity We've discussed this before, I don't think replicating fs errors is a good practice.

1. It's impossible to fully replicate a true native error.
2. If the error is coming from fs-extra, that should be clear, since fs-extra might have a bug.

[ghost]

@RyanZim both good points, and I agree with you completely.

However, the only thing I see as an issue is that in order to check if the error thrown is the already exists error, one would have to search the string. I just thought it may be simpler is an error code or number were added (so users could just check for that rather than searching the string for already exists.