Bug 21786 – install.sh: Add error for trying to install two compilers

A command like ~/dlang/install.sh install ldc,dub installs ldc and a corresponding version of dub. However, install.sh does not support commands here other than dub. For instance, a call like ~/dlang/install.sh install ldc,dmd will return errors like Downloading and unpacking https://github.com/dlang/dub/releases/download/v/dub-v-linux-x86_64.tar.gz curl: (22) The requested URL returned error: 404 Instead, there should be a check that anything after the comma starts with dub and have an error if it is not. Alternately, it could loop through anything comma'ed so that the above command installs both ldc and dmd.

THIS ISSUE HAS BEEN MOVED TO GITHUB https://github.com/dlang/installer/issues/707 DO NOT COMMENT HERE ANYMORE, NOBODY WILL SEE IT, THIS ISSUE HAS BEEN MOVED TO GITHUB