Prevent HTML output in Dropzone when errors occur

A somewhat hackish solution, leveraging the existing behavior in the
error handler, driven by the DISABLE_INLINE_ERROR_REPORTING constant.

When bug_report.php or bugnote_add.php are called via AJAX (as is the
case when Dropzone handles file uploads), the constant is set with a new
value of text. This is picked up by error_handler(), which sends the
error message in the response body as plain text.

Note: AJAX detection relies on presence of non-standard X-Requested-With
header with value 'XMLHttpRequest' (which is set by Dropzone 1).

Fixes 0036303