Issue 25206: fix f-string exceptions to match the code.

pep-0498.txt

@@ -398,15 +398,13 @@ Unmatched braces::
 
   >>> f'x={x'
     File "<stdin>", line 1
-  SyntaxError: missing '}' in format string expression
+  SyntaxError: f-string: expecting '}'
 
 Invalid expressions::
 
   >>> f'x={!x}'
-    File "<fstring>", line 1
-      !x
-      ^
-  SyntaxError: invalid syntax
+    File "<stdin>", line 1
+  SyntaxError: f-string: empty expression not allowed
 
 Run time errors occur when evaluating the expressions inside an
 f-string. Note that an f-string can be evaluated multiple times, and