Fix crashes with unpacking SyntaxError

In general, mypy doesn't promise to be able to check the remainder of
your code in the presence of syntax errors, so just make this a blocking
error.

Fixes python#9137
Fixes python#3825 (most of the reports in this issue were fixed by python#8827)

mypy/semanal.py

@@ -3719,8 +3719,7 @@ def visit_dict_expr(self, expr: DictExpr) -> None:
 
     def visit_star_expr(self, expr: StarExpr) -> None:
         if not expr.valid:
-            # XXX TODO Change this error message
-            self.fail('Can use starred expression only as assignment target', expr)
+            self.fail('Can use starred expression only as assignment target', expr, blocker=True)
         else:
             expr.expr.accept(self)
 

test-data/unit/check-tuples.test

@@ -1002,6 +1002,10 @@ b = (1, 'x')
 a = (0, *b, '')
 [builtins fixtures/tuple.pyi]
 
+[case testUnpackSyntaxError]
+*foo  # E: Can use starred expression only as assignment target
+[builtins fixtures/tuple.pyi]
+
 [case testTupleMeetTupleAny]
 from typing import Union, Tuple
 class A: pass