Check precompile for the asset's logical_path

.. not whatever source we were originally given, which may be missing an
extension.

In doing so, we must also ensure that when we look up an asset from an
extensionless name, we end up with the one we intended -- so
check_errors_for should pass a :type through.

Fixes #128.
Merge pull request #129 from matthewd/precompile_extensions

Check precompile for the asset's logical_path