Bug 1371882 - MEMORYBLOCKCACHE_ERRORS telemetry to catch unexpected errors without crashing - r=cpearce,francois

No errors are expected to happen in MemoryBlockCache (except a few
'InitAllocation', which would still be good to know about), but instead of
taking drastic measures in these cases (i.e., crash), I would prefer to
collect some telemetry first.

MozReview-Commit-ID: 4WdFS34lgzj

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extra : rebase_source : 5600d0b93d4d438d8cc9cf5a74d9fbf24fe2822e