improve error message when invalid CompoundLib gets loaded

Before it just complained that the compound library was created with
an unsupported version of OST while in fact it could have been an empty or
even random file. Now it says:
Exception: Could not read OST version from loaded SQLITE DB - no valid
CompoundLib

modules/conop/src/compound_lib.cc

@@ -235,7 +235,7 @@ String CompoundLib::GetOSTVersionUsed() {
     }
     assert(SQLITE_DONE==sqlite3_step(stmt));
   } else {
-    LOG_WARNING("your compound library might be outdated.");
+    LOG_ERROR(sqlite3_errmsg(db_->ptr));
     sqlite3_finalize(stmt);      
     return String();
   }
@@ -409,6 +409,16 @@ CompoundLibPtr CompoundLib::Load(const String& database, bool readonly)
     LOG_ERROR(sqlite3_errmsg(lib->db_->ptr));
     return CompoundLibPtr();
   }
+
+  // Use ost_version as proxy if the loaded library has anything to do with
+  // what we expect. We don't check for supported version here. This is done
+  // later.
+  String ost_version_used = lib->GetOSTVersionUsed();
+  if(ost_version_used == "") {
+    throw ost::Error("Could not read OST version from loaded SQLITE DB - "
+                     "no valid CompoundLib");
+  }
+
   String aq;
   sqlite3_stmt* stmt;
   // check if SMILES are available