Allow region errors to be dynamic (#10323)

remove other FIXMEs as we are not planning to fix these, 
instead we will add dynamism case by case basis.

fixes #10250

cmd/signature-v4_test.go

@@ -46,7 +46,7 @@ func TestDoesPolicySignatureMatch(t *testing.T) {
 		// (0) It should fail if 'X-Amz-Credential' is missing.
 		{
 			form:     http.Header{},
-			expected: ErrMissingFields,
+			expected: ErrCredMalformed,
 		},
 		// (1) It should fail if the access key is incorrect.
 		{