Print message if literal sampler without sampler map

It prints to stderr, then proceeds to crash.
It's bad, but better than nothing.
diff --git a/lib/SPIRVProducerPass.cpp b/lib/SPIRVProducerPass.cpp
index 4bdb8ce..e5af387 100644
--- a/lib/SPIRVProducerPass.cpp
+++ b/lib/SPIRVProducerPass.cpp
@@ -4269,6 +4269,8 @@
     if (Callee->getName().equals("__translate_sampler_initializer")) {
       // Check that the sampler map was definitely used though.
       if (0 == getSamplerMap().size()) {
+        errs() << "error: kernel uses a literal sampler but option -samplermap "
+                  "has not been specified\n";
         llvm_unreachable("Sampler literal in source without sampler map!");
       }