Fix the message of coverage directory error

If a coverage-report directory conflicts, the error message is
incomplete like the following:

$ nova coverage-start --combine
Coverage collection started
$
$ nova coverage-report --html test01
Report path: /tmp/nova-coverage_Krl__k/test01
$
$ nova coverage-start --combine
Coverage collection started
$ nova coverage-report --html test01
ERROR: Directory conflict: %s already exists (HTTP 400)
$

This patch fixes the message.

Fixes bug #1217315

Change-Id: Ie21b0e87262a3caef36d93711ee96529ec699cec
Merge "Fix the message of coverage directory error"