watchman: ignore some of watchman errors Don't display 'illegal_fstypes' errors. In environments with network filesystems, the error messages are quickly pilling up and polluting outputs. Differential Revision: https://phab.mercurial-scm.org/D5955

watchman: add the possibility to set the exact watchman binary location This is necessary to make rolling releases of new watchman versions across users. Differential Revision: https://phab.mercurial-scm.org/D5954