hy/compiler.py: import only handles HySymbol and HyList, bail on others

When (import) encounters anything but a HySymbol or HyList, raise an
exception, as that is not valid in Hy. Previously, anything other than a
HySymbol or HyList was simply ignored, turning that particular import
into a no-op, which was both wrong and confusing.

Reported-by: Richard Parsons <richard.lee.parsons@gmail.com>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #504 from algernon/h/import-error-reporting

hy/compiler.py: import only handles HySymbol and HyList, bail on others