Handle request errors in copymove

[miquella]

I've been connecting to my Synology's WebDAV and have been encountering semi-regular panics with the following stacktrace:

github.com/studio-b12/gowebdav.(*Client).copymove(0xc0004ec140, 0x12c5bd5, 0x4, 0xc01a132dc0, 0x4e, 0xc0190efe80, 0x3a, 0x1, 0x0, 0x0)
        /home/runner/go/pkg/mod/github.com/studio-b12/gowebdav@v0.0.0-20210203212356-8244b5a5f51a/requests.go:141 +0xbc
github.com/studio-b12/gowebdav.(*Client).Rename(...)
        /home/runner/go/pkg/mod/github.com/studio-b12/gowebdav@v0.0.0-20210203212356-8244b5a5f51a/client.go:302
…

It appears that this is simply due to not properly handling request errors during copy/move.

[chripo]

thank you!!