Confirm that options are valid before starting

When using the 'Sampler' or 'Checkpoint' options, if one of the entered
names has a typo, an error will only be thrown once the `draw_xy_grid`
loop reaches that name. This can waste a lot of time for large grids
with a typo near the end of a list, since the script needs to start over
and re-generate any earlier images to finish making the grid.

Also fixing typo in variable name in `draw_xy_grid`.

scripts/xy_grid.py

@@ -145,7 +145,7 @@ def draw_xy_grid(p, xs, ys, x_labels, y_labels, cell, draw_legend):
     ver_texts = [[images.GridAnnotation(y)] for y in y_labels]
     hor_texts = [[images.GridAnnotation(x)] for x in x_labels]
 
-    first_pocessed = None
+    first_processed = None
 
     state.job_count = len(xs) * len(ys) * p.n_iter
 
@@ -154,8 +154,8 @@ def draw_xy_grid(p, xs, ys, x_labels, y_labels, cell, draw_legend):
             state.job = f"{ix + iy * len(xs) + 1} out of {len(xs) * len(ys)}"
 
             processed = cell(x, y)
-            if first_pocessed is None:
-                first_pocessed = processed
+            if first_processed is None:
+                first_processed = processed
 
             try:
               res.append(processed.images[0])
@@ -166,9 +166,9 @@ def draw_xy_grid(p, xs, ys, x_labels, y_labels, cell, draw_legend):
     if draw_legend:
         grid = images.draw_grid_annotations(grid, res[0].width, res[0].height, hor_texts, ver_texts)
 
-    first_pocessed.images = [grid]
+    first_processed.images = [grid]
 
-    return first_pocessed
+    return first_processed
 
 
 re_range = re.compile(r"\s*([+-]?\s*\d+)\s*-\s*([+-]?\s*\d+)(?:\s*\(([+-]\d+)\s*\))?\s*")
@@ -216,7 +216,6 @@ class Script(scripts.Script):
                     m = re_range.fullmatch(val)
                     mc = re_range_count.fullmatch(val)
                     if m is not None:
-
                         start = int(m.group(1))
                         end = int(m.group(2))+1
                         step = int(m.group(3)) if m.group(3) is not None else 1
@@ -258,6 +257,16 @@ class Script(scripts.Script):
                 valslist = list(permutations(valslist))
 
             valslist = [opt.type(x) for x in valslist]
+            
+            # Confirm options are valid before starting
+            if opt.label == "Sampler":
+                for sampler_val in valslist:
+                    if sampler_val.lower() not in samplers_dict.keys():
+                        raise RuntimeError(f"Unknown sampler: {sampler_val}")
+            elif opt.label == "Checkpoint name":
+                for ckpt_val in valslist:
+                    if modules.sd_models.get_closet_checkpoint_match(ckpt_val) is None:
+                        raise RuntimeError(f"Checkpoint for {ckpt_val} not found")
 
             return valslist