Add more information to errors in ReadBlockFromDisk

A lot of times, disk corruption problems appear here.
To facilitate debugging and troubleshooting, add position information
to the error messages.

src/chain.h

@@ -48,6 +48,12 @@ struct CDiskBlockPos
 
     void SetNull() { nFile = -1; nPos = 0; }
     bool IsNull() const { return (nFile == -1); }
+
+    std::string ToString() const
+    {
+        return strprintf("CBlockDiskPos(nFile=%i, nPos=%i)", nFile, nPos);
+    }
+
 };
 
 enum BlockStatus {

src/main.cpp

@@ -1199,19 +1199,19 @@ bool ReadBlockFromDisk(CBlock& block, const CDiskBlockPos& pos)
     // Open history file to read
     CAutoFile filein(OpenBlockFile(pos, true), SER_DISK, CLIENT_VERSION);
     if (filein.IsNull())
-        return error("ReadBlockFromDisk: OpenBlockFile failed");
+        return error("ReadBlockFromDisk: OpenBlockFile failed for %s", pos.ToString());
 
     // Read block
     try {
         filein >> block;
     }
     catch (const std::exception& e) {
-        return error("%s: Deserialize or I/O error - %s", __func__, e.what());
+        return error("%s: Deserialize or I/O error - %s at %s", __func__, e.what(), pos.ToString());
     }
 
     // Check the header
     if (!CheckProofOfWork(block.GetHash(), block.nBits))
-        return error("ReadBlockFromDisk: Errors in block header");
+        return error("ReadBlockFromDisk: Errors in block header at %s", pos.ToString());
 
     return true;
 }
@@ -1221,7 +1221,8 @@ bool ReadBlockFromDisk(CBlock& block, const CBlockIndex* pindex)
     if (!ReadBlockFromDisk(block, pindex->GetBlockPos()))
         return false;
     if (block.GetHash() != pindex->GetBlockHash())
-        return error("ReadBlockFromDisk(CBlock&, CBlockIndex*): GetHash() doesn't match index");
+        return error("ReadBlockFromDisk(CBlock&, CBlockIndex*): GetHash() doesn't match index for %s at %s",
+                pindex->ToString(), pindex->GetBlockPos().ToString());
     return true;
 }