Improve block database load error reporting

src/main.h

@@ -139,6 +139,8 @@ bool LoadExternalBlockFile(FILE* fileIn, CDiskBlockPos *dbp = NULL);
 bool InitBlockIndex();
 /** Load the block tree and coins database from disk */
 bool LoadBlockIndex();
+/** Unload database information */
+void UnloadBlockIndex();
 /** Verify consistency of the block and coin databases */
 bool VerifyDB();
 /** Print the loaded block tree */