Better error when failing to download the list of packages #150

There was a glitch in the package distro which resulted in pkglist.csv missing on the server.

The PackageManager info messages were completely useless in diagnosing this:

gap> InstallPackage("io");
#I  Getting PackageInfo URLs...
#I  Using curlInterface to download...
#I  Package "io" not found in package list
false

We saw this error in CI in a bunch of places and it was super confusing. Luckily I could reproduce locally, and after tracing through the code, finally discovered that it was trying and failing to download https://github.com/gap-system/PackageDistro/releases/download/latest/pkglist.csv

After that, understanding and fixing the root cause was easy.

But life would have been soooo much easier if after Getting PackageInfo URLs... it would have printed something indicating the error and caus:. Say:

Error, could not obtain package list. Downloading 
   https://github.com/gap-system/PackageDistro/releases/download/latest/pkglist.csv
failed. Perhaps a network error?