Provide less cryptic error for missing systemlib sections (#9566)

Summary:
If a new extension systemlib isn't wired up with the build system, its unit emitters and decls won't be embedded in the HHVM binary, causing a cryptic crash when trying to run Hack code ("Invalid varint value: too few bytes."). One then has to fire up a debugger to determine what systemlib is missing.

Instead, raise a more informative error if a specific systemlib was not found in the binary.

Split from #9564.

Pull Request resolved: #9566

Reviewed By: ricklavoie

Differential Revision: D67874442

fbshipit-source-id: 876fe61ff08fd05a311659a44c2b0a19ea878f94

hphp/runtime/vm/runtime-compiler.cpp

@@ -44,6 +44,7 @@
 #include "hphp/zend/zend-string.h"
 #include "unit-emitter.h"
 
+#include <stdexcept>
 #include <folly/Likely.h>
 #include <folly/Range.h>
 
@@ -259,6 +260,10 @@ Unit* get_systemlib(const std::string& path, const Extension* extension) {
 
   auto buffer = get_embedded_section(path+".ue");
 
+  if (buffer.empty()) {
+    throw std::invalid_argument("Missing systemlib unit emitter for path: " + path);
+  }
+
   UnitEmitterSerdeWrapper uew;
   BlobDecoder decoder(buffer.data(), buffer.size());
   uew.serde(decoder, extension);