Debugging CSS/JavaScript compilation errors can be difficult when viewed
within the context of a Django error page, and if there's a wider
problem with other CSS/JavaScript packages, they won't show up until the
first compilation error is fixed. This is worse if running in a mode
where Pipeline is running but Django error pages aren't being shown.

This change introduces an option, PIPELINE.SHOW_ERRORS_INLINE (now
enabled by default) that outputs any compilation errors at the top of
the page, visually distinguished from the rest of the page, and
containing information on the failing package, command line, and output.
This makes it much easier to debug what went wrong.

The errors are outputted into a hidden div, and then once the page has
fully loaded, they're placed at the top. This allows Pipeline usage
anywhere on the page to promote errors to the top, in the order in which
they're loaded. These also have a default style, but that can be
overridden (and in fact the whole error template overridden) by the
consuming application.