Use `asset_path` with error checking for all calls to `path_to_asset`

Currently, `asset_path` with error checking is aliased to `path_to_asset_with_errors`. This requires having to override `javascript_path`, `javascript_url`, `stylesheet_path`, `stylesheet_url` in order to check if assets accessed using those helpers need to be included in the precompile array (this was not done in #84, which means `asset_path` produces error messages while `javascript_path`, etc. does not). By aliasing the `asset_path` with error checking to `path_to_assets` which those helpers already use, we force the helpers to go through the error checking.

Most of the changes are additions to the tests.