Report findings of possible github zipfile caching in a comment, improve a Sprintf() setup.

/*

While exploring the possibility of local caching of the zip file, I discovered

this command to retrieve the time stamp of the last commit into any particular

repo:

 $time curl -I -i https://api.github.com/repos/KiCad/Mounting_Holes.pretty

This gets just the header to what would otherwise return information on the repo

in JSON format, and is reasonably faster than actually getting the repo

in zip form.  However it still takes 5 seconds or more when github is busy, so

I have lost my enthusiasm for local caching until a faster time stamp retrieval

mechanism can be found, or github gets more servers.  But note that the occasionally

slow response is the exception rather than the norm.  Normally the response is

down around a 1/3 of a second.  The information we would use is in the header

named "Last-Modified" as seen below.  This would need parsing, but avhttp may

offer some help there, if not, then boost async probably does.

HTTP/1.1 200 OK

Server: GitHub.com

Date: Mon, 27 Jan 2014 15:46:46 GMT

Content-Type: application/json; charset=utf-8

Status: 200 OK

X-RateLimit-Limit: 60

X-RateLimit-Remaining: 49

X-RateLimit-Reset: 1390839612

Cache-Control: public, max-age=60, s-maxage=60

Last-Modified: Mon, 02 Dec 2013 10:08:51 GMT

ETag: "3d04d760f469f2516a51a56eac63bbd5"

Vary: Accept

X-GitHub-Media-Type: github.beta

X-Content-Type-Options: nosniff

Content-Length: 6310

Access-Control-Allow-Credentials: true

Access-Control-Expose-Headers: ETag, Link, X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset, X-OAuth-Scopes, X-Accepted-OAuth-Scopes, X-Poll-Interval

Access-Control-Allow-Origin: *

X-GitHub-Request-Id: 411087C2:659E:50FD6E6:52E67F66

Vary: Accept-Encoding

*/

    catch( boost::system::system_error& e )

    {

        // https "GET" has faild, report this to API caller.

        wxString errorcmd( wxT("https GET command failed") ); // Do not translate this message

        wxString fmt( _( "%s\nCannot get/download Zip archive: '%s'\nfor library path: '%s'.\nReason: '%s'" ) );

        static const char errorcmd[] = "https GET command failed";  // Do not translate this message

        UTF8 fmt( _( "%s\nCannot get/download Zip archive: '%s'\nfor library path: '%s'.\nReason: '%s'" ) );

        string msg = StrPrintf( TO_UTF8( fmt ),

                TO_UTF8( errorcmd ),

                zip_url.c_str(),

                TO_UTF8( aRepoURL ),

        string msg = StrPrintf( fmt.c_str(),

                errorcmd,

                // Report both secret zip_url and Lib Path, to user.  The secret

                // zip_url may go bad at some point in future if github changes

                // their server architecture.  Then fix repoURL_zipURL() to reflect

                // new architecture.

                zip_url.c_str(), TO_UTF8( aRepoURL ),

                e.what() );

        THROW_IO_ERROR( msg );

   <p>It could have used version 3 of the github.com API documented here:

   <pre>

       http://developer.github.com

       http://developer.github.com/v3/

       https://help.github.com/articles/creating-an-access-token-for-command-line-use

   </pre>