2.0 | Namespaces::getDeclaredName(): rethink return value

[jrfnl]

As things are, the Namespaces::getDeclaredName()method will return string|false and not throw exceptions for invalid input received.

I think this should be rethought and exceptions should be thrown for invalid input, like a non-integer stack pointer, a non-existent stack pointer, a non-T_NAMESPACE stack pointer.

To be decided: what to do with namespace keywords which are not for a namespace declaration, i.e. the namespace in namespace\functionName().
Then again, by the time, we're talking v 2.0, support for PHPCS < 4.0 will likely have been dropped and this should no longer be an issue as namespace relative names are no longer tokenized as T_NAMESPACE + T_NS_SEPARATOR + T_STRING in PHPCS 4.0 anyway.

Loosely related to #599