Compaction: Report a useful error message when temporary files cannot be created

The couch_compact program appears to create a number of temporary files (using tmpdir()) during compaction (at a guess for compacting the b-tree index). If these files cannot be created compaction fails with a generic error message:

[couchdb:info,2014-06-26T1:06:41.785,ns_1@localhost:<0.29571.6153>:couch_log:info:39]Native compactor output: Couchstore error: no such file

It would be extremely useful if the was more meaningful, and actually pointed out the reason for the failure.

Steps to reproduce:

Run couch_compact with the attached couchdb file (note: just contains test data) with /tmp set to be unwritable to the couchbase user.

$ chmod o-w /tmp
$ sudo -u couchbase -s
$ /opt/couchbase/bin/couch_compact 0.couch.1 foo.out
Couchstore error: no such file