Preliminary local exception refactoring in C++

[bernardnormier]

This is a first step in refactoring the local exceptions in C++:

• removed IceUtil::Exception - the base class for all exceptions is now Ice::Exception
• all local exceptions derive from Ice::LocalException
• Ice::Exception/LocalException/UserException are read-only after construction
• the error message returned by what can be set in the constructor
• the constructors of the refactored exceptions don't allocate memory
• the refactored exceptions have a noexcept copy ctor and assignment operator
• refactored so far: the 3 NotExist and the 3 Unknown exceptions, in addition to the base exceptions
• ice_id() on Ice::Exception now returns a const char* for compatibility with what()

[pepone]

Need to fix this

test/IceGrid/fileLock/test.py
17:        registry.expect(current, ".*IceUtil::FileLockException.*")

[bernardnormier]

Need to rework this update.

[bernardnormier]

This updated PR reworks Ice::Exception and Ice::RequestFailedException.

They have now noexcept copy ctor and assignment operators.