Better error message.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@94544 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CompilerDriver/CompilationGraph.cpp b/lib/CompilerDriver/CompilationGraph.cpp
index 56e5b9c..524607b 100644
--- a/lib/CompilerDriver/CompilationGraph.cpp
+++ b/lib/CompilerDriver/CompilationGraph.cpp
@@ -33,9 +33,11 @@
 namespace llvmc {
 
   const std::string& LanguageMap::GetLanguage(const sys::Path& File) const {
-    LanguageMap::const_iterator Lang = this->find(File.getSuffix());
+    StringRef suf = File.getSuffix();
+    LanguageMap::const_iterator Lang = this->find(suf);
     if (Lang == this->end())
-      throw std::runtime_error(("Unknown suffix: " + File.getSuffix()).str());
+      throw std::runtime_error("File '" + File.str() +
+                                "' has unknown suffix '" + suf.str() + '\'');
     return Lang->second;
   }
 }