run cleanup steps in a finally block

tzk/__main__.py

@@ -211,7 +211,9 @@ class BuildCommand(CliCommand):
         print(f"tzk: Starting build of product '{args.product}'.")
         print(f"tzk: Found {len(steps)} build {numerize(len(steps), 'step')}.")
 
+        failed = False
         # For each build step...
+        try:
             for idx, step in enumerate(steps, 1):
                 # Explain what we're doing. Use first line of the builder's docstring
                 # as a summary, if present.
@@ -227,26 +229,32 @@ class BuildCommand(CliCommand):
                     continue
 
                 # Execute step and handle any errors.
-            try:
                 step()
+
         except BuildError as e:
+            failed = True
             print(f"tzk: ERROR: {str(e)}")
             print(f"tzk: Build of product '{args.product}' failed on step {idx}, "
                     f"backed by builder '{step.__name__}'. ")
-                sys.exit(1)
+            print(f"tzk: Add '--skip-builder {step.__name__}' if you'd like "
+                    f"to skip this step.")
+
         except Exception:
+            failed = True
             print(f"tzk: Build of product '{args.product}' failed on step {idx}: "
                     f"unhandled exception. "
                     f"The original error follows:")
             traceback.print_exc()
-                sys.exit(1)
 
-        # TODO: This should run in a finally() block; leaving for now as it's convenient for development :)
+        finally:
             for idx, step in enumerate(steps, 1):
                 if hasattr(step, 'cleaner'):
-                print(f"tzk: Running cleanup routine for step {idx}...")
+                    print(f"tzk: Running cleanup routine registered by step {idx}...")
                     step.cleaner()
 
+        if failed:
+            sys.exit(1)
+        else:
             print(f"tzk: Build of product '{args.product}' completed successfully.")