Github plugin: fix lack of message when trying to download a non existing footprint library (Curl does not return an error: the message "Not Fount" is stored in buffer, and the zip handler returns a not suitable error message)

Minor and very minor other fixes.

pcbnew/github/github_plugin.cpp

@@ -38,7 +38,7 @@ 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. 
+named "Last-Modified" as seen below.
 
 
 HTTP/1.1 200 OK
@@ -545,13 +545,23 @@ void GITHUB_PLUGIN::remoteGetZip( const wxString& aRepoURL ) throw( IO_ERROR )
     {
         UTF8 fmt( _( "%s\nCannot get/download Zip archive: '%s'\nfor library path: '%s'.\nReason: '%s'" ) );
 
-        std::string msg = StrPrintf( fmt.c_str(), 
-                                     zip_url.c_str(), 
+        std::string msg = StrPrintf( fmt.c_str(),
+                                     zip_url.c_str(),
                                      TO_UTF8( aRepoURL ),
                                      TO_UTF8( ioe.errorText ) );
 
         THROW_IO_ERROR( msg );
     }
+
+    // If the zip archive is not existing, the received data is "Not Found",
+    // and no error is returned by kcurl.Perform().
+    if( m_zip_image.compare( 0, 9, "Not Found" ) == 0 )
+    {
+        UTF8 fmt( _( "Cannot download library '%s'.\nThe library does not exist on the server" ) );
+        std::string msg = StrPrintf( fmt.c_str(), TO_UTF8( aRepoURL ) );
+
+        THROW_IO_ERROR( msg );
+    }
 }
 
 #if 0 && defined(STANDALONE)