check-templates: Insist on 4-space indents.

We now complain about programmers who don't use
4-space indents in template files, rather than
letting the pretty printer fix them.

This is partly just to simplify the pretty printer
code (in future commits), but it also makes the
symptom more obvious to newbie developers. They
are probably just as able to react to the direct
error messages as they are able to figure out how
to read diffs from the pretty printer and grok
the --fix syntax. And once they learn the convention
and configure their editor, it should then be a
one time problem.

tools/lib/template_parser.py

@@ -453,6 +453,19 @@ def prevent_whitespace_violations(fn: str, tokens: List[Token]) -> None:
                     f"""Please just make row {token.line} in {fn} a truly blank line (no spaces)."""
                 )
 
+            if len(token.s) % 4 != 0:
+                raise TemplateParserException(
+                    f"""
+                        Please use 4-space indents for template files. Most of our
+                        codebase (including Python and JavaScript) uses 4-space indents,
+                        so it's worth investing in configuring your editor to use
+                        4-space indents for files like
+                        {fn}
+
+                        The line at row {token.line} is indented with {len(token.s)} spaces.
+                    """
+                )
+
         if token.kind == "whitespace":
             if len(token.s) > 1:
                 raise TemplateParserException(