Clarify thrown Exceptions in `SessionSettings`

[chrjohn]

E.g. there is

https://github.com/quickfix-j/quickfixj/blob/895224da1a05055bc99d936920d1e1ed7edc9935/quickfixj-core/src/main/java/quickfix/SessionSettings.java#L368-L374

but FieldConvertError is never thrown but wrapped into ConfigError.

Then there is

https://github.com/quickfix-j/quickfixj/blob/895224da1a05055bc99d936920d1e1ed7edc9935/quickfixj-core/src/main/java/quickfix/SessionSettings.java#L306-L312

where ConfigError could be thrown by getString.

I think getBool (and other methods that have this behaviour) should be changed to not mask the FieldConvertError so that it is clear whether there is a problem with the key (i.e. setting was not found) or the value (i.e. value could not be converted).