Platform: improve WindowlessEglApp error with a not found CUDA device.

Say how many devices there were in total, so it's possible to
distinguish the case of trying to find a CUDA device on a machine with
no NVidia GPU (where it says it found some other EGL devices) and the
case of drivers being completely broken for some reason (where it says
it didn't find any device at all).

src/Magnum/Platform/WindowlessEglApplication.cpp

@@ -230,7 +230,7 @@ WindowlessEglContext::WindowlessEglContext(const Configuration& configuration, G
                 /* None found */
                 if(selectedDevice == UnsignedInt(count)) {
                     Error e;
-                    e << "Platform::WindowlessEglApplication::tryCreateContext(): unable to find EGL device for CUDA device" << configuration.cudaDevice();
+                    e << "Platform::WindowlessEglApplication::tryCreateContext(): unable to find CUDA device" << configuration.cudaDevice() << "among" << count << "EGL devices in total";
                     return;
                 }