Fix PHP Fatal error: Undefined class constant 'ERROR_FORBIDDEN' (T2253)

Summary: Fixes T2253

Test Plan:
With enabled WebDAV browser upload the same file twice to the same folder.

Second try should return proper exception status instead of a white page.

Reviewers: #irony_developers

Maniphest Tasks: T2253

Differential Revision: https://git.kolab.org/D384