[tree] Include branch name in an error message.

Pointed out by Philippe in #18259.

Author: hageboeck

tree/treeplayer/src/TTreeReaderValue.cxx

@@ -833,8 +833,9 @@ void ROOT::Internal::TTreeReaderValueBase::ErrorAboutMissingProxyIfNeeded()
    // Print the error only if the branch name does not appear in the list of
    // missing proxies that the user explicitly requested not to error about
    if (!fTreeReader || fTreeReader->fMissingProxies.count(fBranchName.Data()) == 0)
-      Error("TTreeReaderValue::Get()", "Value reader not properly initialized, did you call "
-                                       "TTreeReader::Set(Next)Entry() or TTreeReader::Next()?");
+      Error("TTreeReaderValue::Get()", "Value reader for branch " + fBranchName +
+                                          " not properly initialized, did you call "
+                                          "TTreeReader::Set(Next)Entry() or TTreeReader::Next()?");
 }
 
 namespace cling {