added path checking for input file and exit codes

src/exit_code.h

@@ -0,0 +1,9 @@
+typedef enum {
+    EXIT_OK = 0,
+    ERR_ARGUMENT = 1,
+    ERR_FILE = 2,
+    ERR_DATABASE = 3,
+    ERR_CONNECTION = 4,
+    ERR_MEMORY = 5,
+    ERR_QUERY = 6,
+} exit_code;