Again trouble starting OSCAR while offline due to GAP issues

[fingolfin]

Reported by @ederc (who'll provide details if he still has them).

Ideal solution is to finally get GAP_pkg_*.jl out of the door....

[ederc]

Here is the error message:

julia> using Oscar
ERROR: InitError: Error thrown by GAP: Error, RequestError: Could not resolve host: raw.githubusercontent.com while requesting https://raw.githubusercontent.com/gap-system/gap-distribution/master/DistributionUpdate/PackageUpdate/currentPackageInfoURLList in
  Julia.Core._apply( julia_obj, args ) at /Users/ederc/repos/julia-depot/packages/GAP/p0xxt/pkg/JuliaInterface/gap/calls.gi:15 called from
PKGMAN_DownloadURL( PKGMAN_PackageInfoURLList ) at /Users/ederc/repos/julia-depot/scratchspaces/c863536a-3901-11e9-33e7-d5cd0df7b904/gap_3622292053906931894_1.10/pkg/packagemanager/gap/PackageManager.gi:19 called from
GetPackageURLs(  ) at /Users/ederc/repos/julia-depot/scratchspaces/c863536a-3901-11e9-33e7-d5cd0df7b904/gap_3622292053906931894_1.10/pkg/packagemanager/gap/PackageManager.gi:125 called from
InstallPackageFromName( string, version, interactive ) at /Users/ederc/repos/julia-depot/scratchspaces/c863536a-3901-11e9-33e7-d5cd0df7b904/gap_3622292053906931894_1.10/pkg/packagemanager/gap/PackageManager.gi:82 called from
<function "InstallPackage">( <arguments> )
 called from read-eval loop at *defin*:0

Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] ThrowObserver(depth::Int32)
   @ GAP ~/repos/julia-depot/packages/GAP/p0xxt/src/GAP.jl:88
during initialization of module GaloisGrp

[ThomasBreuer]

This means that we are in the __init__ function of the GaloisGrp module in experimental/GaloisGrp when the error happens.
The problem is thus in the call GAP.Packages.load("ferret"; install=true), that is, GAP's ferret package cannot be loaded. Then an installation of ferret is tried, which fails because the package archive cannot be downloaded.

My understanding was that once the ferret package has been successfully installed, this problem should not occur anymore.

An explanation for the problem would be that the installation of ferret does not work when one is online, but one does not run into an error because the download works, and the __init__ function does not check the return value of the GAP.Packages.load call.
As soon as one is offline, one runs into the error.

[ThomasBreuer]

It seems that we have to catch errors like the one reported here.
The message above gives the impression that GAP throws the error. However, the function in question is the Julia function Downloads.download, thus the problem can/must be fixed in GAP.jl.
This function itself does not admit catching the error via an optional argument, but it calls Downloads.request, which supports such a keyword argument (throw = false). Alternatively, we could just use try/catch.

I am working on a pull request for that.