Escape HTML characters when printing the train

Since it's wrapped in a `<pre>`, I assume it's meant to be inserted into
HTML?

tilde-train.py

@@ -41,6 +41,7 @@ import curses
 from signal import signal, SIGINT
 import time # allowing the loop steps of train animation to be slowed
 import string # for input validation
+from html import escape
 from inspect import cleandoc
 from pathlib import Path
 
@@ -90,6 +91,9 @@ default_car = [
   r"   \__/ \__/      \__/ \__/   ",
 ]
 
+class CarError(Exception):
+  """Error related validating a car."""
+
 def print_help():
   print(
     cleandoc(
@@ -206,9 +210,11 @@ def validate_car(car: list[str]):
   while car_list[(len(car_list)-1)].strip() == "":
     car_list.pop() # clear bottom
 
-  # len(choochoo_list) is the height of the train car, in number of rows.
-  if len(car_list) > max_y+1 or len(car_list) == 0:
-    return 0  # train car too tall or non-existant; skip it.
+  # len(car_list) is the height of the train car, in number of rows.
+  if len(car_list) > max_y+1:
+    raise CarError(f"Car is too tall ({len(car_list)} > {max_y + 1}).")
+  if len(car_list) == 0:
+    raise CarError(f"Car is empty.")
 
   # vertically pad short cars with 1 space (lines will be lengthened later).
   while len(car_list) < max_y:
@@ -221,9 +227,9 @@ def validate_car(car: list[str]):
 
   for idx,row in enumerate(car_list):
     if len(row) > max_x+1: # check length of each row in .choochoo file.
-      return 0 # train car too long; skip it.
+      raise CarError(f"Car is too wide ({len(row)} > {max_x + 1}).")
     elif "\t" in row or "\v" in row or "\f" in row or "\r" in row:
-      return 0 # skip if contains tabs, vert tabs, line feeds or carriage ret
+      raise CarError(f"Car contains illegal control characters.")
     elif len(row) < longest_line:
       padding = " "*(longest_line - len(row))
       car_list[idx] += padding # add padding spaces.
@@ -241,13 +247,12 @@ def print_all_cars():
         continue # the train car was too tall; skip it.
 
       car = validate_car(choochoo_list) # printing is only a DEBUG feature.
-      if car != 0:
-        print("")
-        print(fname + ":")
-        print("\n".join(car)) # print the car to stdout
+      print("")
+      print(fname + ":")
+      print("\n".join(car)) # print the car to stdout
 
     except:
-      pass;
+      pass
 
 
 def chuggachugga(stdscr: curses.window):
@@ -306,11 +311,10 @@ for fname in glob.glob('/home/*/' + traincarFN):
       continue # the train car was too tall; skip it.
 
     car = validate_car(choochoo_list) # printing is only a DEBUG feature.
-    if car != 0:
-      cars.append(car) # start a list of lists (list of cars) here.
+    cars.append(car) # start a list of lists (list of cars) here.
 
   except:
-    pass;
+    pass
 
 while len(cars) < max_cars:
   cars.append([*default_car]) # add copies of default cars if train too short
@@ -336,7 +340,7 @@ train_str = "\n".join(train)
 
 if print_train:
   print("<pre>")
-  print(train_str)
+  print(escape(train_str))
   print("</pre>")
   sys.exit()