tablet: use RETURN_NOT_OK_PREPEND in more places in compaction code
tablet: use RETURN_NOT_OK_PREPEND in more places in compaction code

I was trying to debug some compaction-related issues and found that it was
difficult to see where things went wrong, because status messages were
propagated without any context prepended. Simple patch adds more context
messages to the errors.

Change-Id: Iafd010d1789a3a41b4dd1e245cbd26a6e0337045
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1462
Reviewed-by: Jean-Daniel Cryans <4bf4c125525b8623ac45dfd7774cbf531df19085@cloudera.com>
Tested-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>

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