[clangd] Explicitly fail if the file passed to --check is not valid.

clang-tools-extra/clangd/tool/ClangdMain.cpp

if (TweakList.getNumOccurrences())
       return llvm::is_contained(TweakList, T.id());
     return true;
   };
   if (ForceOffsetEncoding != OffsetEncoding::UnsupportedEncoding)
     Opts.Encoding = ForceOffsetEncoding;
 
   if (CheckFile.getNumOccurrences()) {
     llvm::SmallString<256> Path;
-    llvm::sys::fs::real_path(CheckFile, Path, /*expand_tilde=*/true);
+    if (auto Error =
+            llvm::sys::fs::real_path(CheckFile, Path, /*expand_tilde=*/true)) {
+      elog("Failed to resolve path {0}: {1}", CheckFile, Error.message());
+      return 1;
+    }
     log("Entering check mode (no LSP server)");
     uint32_t Begin = 0, End = std::numeric_limits<uint32_t>::max();
     if (!CheckFileLines.empty()) {
       StringRef RangeStr(CheckFileLines);
       bool ParseError = RangeStr.consumeInteger(0, Begin);
       if (RangeStr.empty()) {
         End = Begin;
       } else {