windows: raise a custom exception when static_replace_in_file fails

I knew I would come to regret use of sys.exit().

Author: indygreg

cpython-windows/build.py

@@ -213,6 +213,10 @@ def find_vctools_path():
     return tools_path
 
 
+class NoSearchStringError(Exception):
+    """Represents a missing search string when replacing content in a file."""
+
+
 def static_replace_in_file(p: pathlib.Path, search, replace):
     """Replace occurrences of a string in a file.
 
@@ -225,8 +229,7 @@ def static_replace_in_file(p: pathlib.Path, search, replace):
     # Build should be as deterministic as possible. Assert that wanted changes
     # actually occur.
     if search not in data:
-        log("search string (%s) not in %s" % (search, p))
-        sys.exit(1)
+        raise NoSearchStringError("search string (%s) not in %s" % (search, p))
 
     log("replacing `%s` with `%s` in %s" % (search, replace, p))
     data = data.replace(search, replace)