Exit with nonzero status on error

src/image.c

@@ -1629,7 +1629,7 @@ image load_image_stb(char *filename, int channels)
     unsigned char *data = stbi_load(filename, &w, &h, &c, channels);
     if (!data) {
         fprintf(stderr, "Cannot load image \"%s\"\nSTB Reason: %s\n", filename, stbi_failure_reason());
-        exit(0);
+        exit(1);
     }
     if(channels) c = channels;
     int i,j,k;

src/utils.c

@@ -208,19 +208,19 @@ void error(const char *s)
 {
     perror(s);
     assert(0);
-    exit(-1);
+    exit(1);
 }
 
 void malloc_error()
 {
     fprintf(stderr, "Malloc error\n");
-    exit(-1);
+    exit(1);
 }
 
 void file_error(char *s)
 {
     fprintf(stderr, "Couldn't open file: %s\n", s);
-    exit(0);
+    exit(1);
 }
 
 list *split_str(char *s, char delim)