IQA-463: Add apr_file_open(APR_CREATE) flag for downloaded installer.

This handles the case when the target file doesn't exist, just as APR_TRUNCATE
handles the case when it does.
Strengthen error checks concerning downloaded installer file from
ll_apr_warn_status() to ll_apr_assert_status(). Failing to recognize (e.g.)
failure to open that file only leads to mysterious crashes down the road; this
removes the mystery.