gowebdav library should not hush up errors

[MrVine]

If we carefully see on following methods:

• client.mkcol(), client.Mkdir(), client.MkdirAll()
• client.copymove(), client.doCopyMove()
• client.Write() and client.WriteStream()

• it suppress real errors and returns only status code in the best case.

I propose to change this behaviour to allow user of our library to get original error message.

[MrVine]

@chripo, what do you think about?

[chripo]

the lower case methods are not going to be exported.
therefore error handling / result handling by status code is simpler.
surely there is space for improvement and consistency, in uppercase methods too!
maybe we should introduce an own error structure that includes the status code, without or with the origin error.
simplicity first. ;)

[MrVine]

I have experience of using gowebdav with WebDav server which had outdated certificate, and it was difficult for me to answer the question "why my application does not recieves/sends data?", because all what I got back from gowebdav was:

Mkdir /path/to/my/file.txt 400

When I fix error suppressing and got this error:

x509: certificate signed by unknown authority

I understand where the problem is and fix it quickly.

That is why I propose to return error messages even from lower case methods.
Mkdir 400 it is not an good answer, users should knows how the real error sounds to be able to quickly fix it.

[chripo]

Indeed. Maybe we should wrap error and status code for all exported methods.

[chripo]

We're going to fix this long outstanding issue due to @fischor and @zekroTJA investigations.
Thank you!

[chripo]

finally done.
thanks to @fischor and @zekroTJA