Abort gracefully if internet resource is not available

[ablaette]

The cwbtools package was removed from CRAN because the following aspect of CRAN policy was not met:

Packages which use Internet resources should fail gracefully with an informative message
if the resource is not available or has changed (and not give a check warning nor error).

As far as I can see, the following line in corpus_install() violates the CRAN policy:

if (http_error(tarball)) stop("tarball is not available")

[ablaette]

The function will now abort returning FALSE if a tarball is not available.