Fixed bug where the expression system relied upon the ``str()``

Fixed bug where the expression system relied upon the ``str()``
form of a some expressions when referring to the ``.c`` collection
on a ``select()`` construct, but the ``str()`` form isn't available
since the element relies on dialect-specific compilation constructs,
notably the ``__getitem__()`` operator as used with a Postgresql
``ARRAY`` element.  The fix also adds a new exception class
:class:`.UnsupportedCompilationError` which is raised in those cases
where a compiler is asked to compile something it doesn't know
how to.
[ticket:2780]