* Do not build exception instances in advance
When no `cause` and no clean context of the exception thrown,
the code place where an exception has been created is confusing.
* Fix `SimpleMessageStore` to create an `out of capacity` exception in the
`addMessagesToGroup()` exactly at the point where it is thrown
* Fix `FunctionExpression` and `SupplierExpression` do not pre-create `readOnlyException`:
it is unlikely these kind of expressions are going to be used in the SpEL `write` context,
so we save some time and memory not creating extra object and the plain call `throw new`
gives us a clean context what method call has ended up with such an exception
* * Extract `outOfCapacityException` to the method in the `SimpleMessageStore`
to avoid duplication.
Even if we got in the end extra line in th stack trace, it is still clear from
where it is thrown:
```
org.springframework.messaging.MessagingException: SimpleMessageStore was out of capacity (1) for group 'foo', try constructing it with a larger number.
at org.springframework.integration.store.SimpleMessageStore.outOfCapacityException(SimpleMessageStore.java:324)
at org.springframework.integration.store.SimpleMessageStore.addMessagesToGroup(SimpleMessageStore.java:302)
at org.springframework.integration.store.AbstractMessageGroupStore.addMessageToGroup(AbstractMessageGroupStore.java:189)
at org.springframework.integration.store.SimpleMessageStoreTests.shouldNotHoldMoreThanGroupCapacity(SimpleMessageStoreTests.java:128)
```
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