Provide a better error message

src/main/java/org/leolo/map/osm/extract/ExtractElement.java

@@ -115,7 +115,7 @@ public class ExtractElement {
             PrintStream outputStream = outputFilePath==null?System.out:new PrintStream(new File(outputFilePath));
             File dbFile = new File(dbFilePath);
             if(!dbFile.exists() || !dbFile.canRead()){
-                log.atError().log("Unable to read db file {}", dbFilePath);
+                log.atError().log("Unable to read db file {}, file cannot be read or does not exists", dbFilePath);
                 System.exit(2);
             }
             File actionFile = new File(actionFilePath);