Convert Exception::getMessage() result to string

We specify that the return type of Exception::getMessage() is a
string. However, we don't currently ensure this, because
Exception::$

Convert Exception::getMessage() result to string

We specify that the return type of Exception::getMessage() is a
string. However, we don't currently ensure this, because
Exception::$message is a protected member that can be set to any
type. Fix this by performing an explicit type-cast.

This also requires a temporary refcount increment in the __toString()
object handler, because there is no additional owner of the object,
and it may get released prematurely as part of the __toString() call.

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