Including an HTML file for vcl_error

[James Pearson]

In vcl_error, I can use the synthetic keyword to construct a nice HTML error
page.  For consistency reasons, I'd like this to use the same error page we use
in a few other parts of the stack.  Is there any way to pull in another file as
a value?  I know I can include other VCL files, but if it's valid VCL, then I
can't serve it from e.g. Apache. ;)

Side note: searching for 'varnish synthetic' gets terrible results.