[clangd] Add more error details on the remote index server side

[kadircet]

LGTM

clang-tools-extra/clangd/index/remote/marshalling/Marshalling.cpp

322

i would put this in quotes to make empty strings more easy to detect, i.e. '{0}', same for others.

also s/given/received/ ?

328

why not print the real RemoteIndexRoot here ?

[kbobyrev]

Resolve post-LGTM comments.

clang-tools-extra/clangd/index/remote/marshalling/Marshalling.cpp

 #include "index/SymbolOrigin.h"
 #include "support/Logger.h"
 #include "clang/Index/IndexSymbol.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Error.h"
+#include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/StringSaver.h"
 
 namespace clang {
 namespace clangd {
 namespace remote {
 
 namespace {

 llvm::Expected<std::string>
 Marshaller::relativePathToURI(llvm::StringRef RelativePath) {
   assert(LocalIndexRoot);
   assert(RelativePath == llvm::sys::path::convert_to_slash(
                              RelativePath, llvm::sys::path::Style::posix));
   if (RelativePath.empty())
     return makeStringError("Empty relative path.");
   if (llvm::sys::path::is_absolute(RelativePath))
-    return makeStringError("RelativePath is absolute.");
+    return makeStringError(
+        llvm::formatv("RelativePath '{0}' is absolute.", RelativePath).str());
   llvm::SmallString<256> FullPath = llvm::StringRef(*LocalIndexRoot);
   llvm::sys::path::append(FullPath, RelativePath);
   auto Result = URI::createFile(FullPath);
   return Result.toString();
 }
 
 llvm::Expected<std::string> Marshaller::uriToRelativePath(llvm::StringRef URI) {
   assert(RemoteIndexRoot);
   auto ParsedURI = URI::parse(URI);
   if (!ParsedURI)
     return ParsedURI.takeError();
   if (ParsedURI->scheme() != "file")
-    return makeStringError("Can not use URI schemes other than file.");
+    return makeStringError(
+        llvm::formatv("Can not use URI schemes other than file, given: '{0}'.",
+                      URI)
+            .str());
   llvm::SmallString<256> Result = ParsedURI->body();
   if (!llvm::sys::path::replace_path_prefix(Result, *RemoteIndexRoot, ""))
-    return makeStringError("File path doesn't start with RemoteIndexRoot.");
+    return makeStringError(
+        llvm::formatv("File path '{0}' doesn't start with '{1}'.", Result.str(),
+                      *RemoteIndexRoot)
+            .str());
   // Make sure the result has UNIX slashes.
   return llvm::sys::path::convert_to_slash(Result,
                                            llvm::sys::path::Style::posix);
 }
 
 clangd::SymbolLocation::Position
 Marshaller::fromProtobuf(const Position &Message) {
   clangd::SymbolLocation::Position Result;