Add additional Error log messages

Added error log messages to the vector read code to help track down the cause of FileReadErrors in the integration builds.

IOPool/TFileAdaptor/src/TStorageFactoryFile.cc

@@ -414,6 +414,7 @@ TStorageFactoryFile::ReadBuffersSync(char *buf, Long64_t *pos, Int_t *len, Int_t
     std::vector<IOPosBuffer> &iov = repacker.iov();
     IOSize result = storage_->readv(&iov[0], iov.size());
     if (result != io_buffer_used) {
+      Error("ReadBuffersSync","Storage::readv returned different size result=%ld expected=%ld",result,io_buffer_used);
       return kTRUE;
     }
     xstats.tick(io_buffer_used);
@@ -477,7 +478,13 @@ TStorageFactoryFile::ReadBuffers(char *buf, Long64_t *pos, Int_t *len, Int_t nbu
   astats.tick(total);
 
   // If it didn't suceeed, pass down to the base class.
-  return success ? kFALSE : TFile::ReadBuffers(buf, pos, len, nbuf);
+  if(not success) {
+    if(TFile::ReadBuffers(buf, pos, len, nbuf)) {
+      Error("ReadBuffers", "call to TFile::ReadBuffers failed after prefetch already failed.");
+      return kTRUE;
+    }
+  }
+  return kFALSE;
 }
 
 Bool_t