STObject::applyTemplate() throws with description of error:

The `STObject` member function `setType()` has been renamed to
applyTemplate() and modified to throw if there is a template
mismatch.

The error description in the exception is, in certain cases,
used, to better indicate why a particular transaction was
considered ill formed.

Fixes #2585.

src/test/protocol/STObject_test.cpp

@@ -363,7 +363,7 @@ public:
 
             BEAST_EXPECT(st[sf1] == 1);
             BEAST_EXPECT(st[sf2] == 2);
-            except<missing_field_error>([&]()
+            except<STObject::FieldErr>([&]()
                 { st[sf3]; });
             BEAST_EXPECT(*st[~sf1] == 1);
             BEAST_EXPECT(*st[~sf2] == 2);