PostgreSQL server integrity after losing a tablespace
I can't describe this better than the manual does


  Warning
  
  Even though located outside the main PostgreSQL data directory, tablespaces are an integral part of the database cluster and cannot be treated as an autonomous collection of data files. They are dependent on metadata contained in the main data directory, and therefore cannot be attached to a different database cluster or backed up individually. Similarly, if you lose a tablespace (file deletion, disk failure, etc), the database cluster might become unreadable or unable to start. Placing a tablespace on a temporary file system like a RAM disk risks the reliability of the entire cluster.


Or this blog post


  You can’t just throw a tablespace away and have the rest of the database system keep on working.
  
  Most importantly, the write-ahead log (WAL) that provides PostgreSQL’s crash safety is stored in pg_xlog, and is shared across all tablespaces. PostgreSQL expects to be able to replay this log in order and without errors after a crash or shutdown. Until the log has replayed, the database is assumed to be in an unsafe state and connections will be refused