exo: Remove unnecessary wl_*_post_no_memory calls.
exo: Remove unnecessary wl_*_post_no_memory calls.

Chromium will crash when OOM. This removes all wl_*_post_no_memory
calls that can't happen as a result of that.

This also replaces the post_no_memory call in surface_attach with
DLOG warning and sends more appropriate error messages to clients
when failing to assign roles to surfaces.

BUG=549781
TEST=exo_unittests

Review URL: https://codereview.chromium.org/1717773002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#376579}

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