Error: track message and prefix/location separately. (#1529)

This allows us to combine multiple Errors together without repeating the prefix
information. Also fixes several cases where two "COMPILATION ERROR" prefixes
would be prepended to the same message when errors with prefixes and locations
were produced by the lexer and parser.

explorer/main.cpp

@@ -89,7 +89,7 @@ static auto Main(llvm::StringRef default_prelude_file, int argc, char* argv[])
 auto ExplorerMain(llvm::StringRef default_prelude_file, int argc, char** argv)
     -> int {
   if (auto result = Main(default_prelude_file, argc, argv); !result.ok()) {
-    llvm::errs() << result.error().message() << "\n";
+    llvm::errs() << result.error() << "\n";
     return EXIT_FAILURE;
   }
   return EXIT_SUCCESS;