Error if any dbupgrade src points to anything but a file.
Error if any dbupgrade src points to anything but a file.

    % dbupgrade /doesnotexist /tmp/
    Error:  Source must point to existing main DB files

    Usage:  dbupgrade /path/to/srcdb [...] /path/to/dest

    src is the path to the main database.  You can specify more than one.
    dest can be either a new main database filename, or a directory
    (the old DB name will be used).

    If more than one src db is specified, the destination *must* be a
    directory.

Change-Id: Id0b22a6d413b5e1618c674c052b6c7d03c84ba6a
Reviewed-on: http://review.membase.org/4314
Tested-by: Bin Cui <ed18693fff32c00e22495f4877a3b901bed09041@gmail.com>
Reviewed-by: Bin Cui <ed18693fff32c00e22495f4877a3b901bed09041@gmail.com>

jimwwalker/ep-engine,abhinavdangeti/ep-engine,couchbase/ep-engine,daverigby/kv_engine,daverigby/ep-engine,daverigby/ep-engine,hisundar/ep-engine,zbase/ep-engine,zbase/ep-engine,membase/ep-engine,hisundar/ep-engine,sriganes/ep-engine,sriganes/ep-engine,daverigby/kv_engine,jimwwalker/ep-engine,zbase/ep-engine,abhinavdangeti/ep-engine,membase/ep-engine,abhinavdangeti/ep-engine,sriganes/ep-engine,couchbaselabs/ep-engine,teligent-ru/ep-engine,zbase/ep-engine,abhinavdangeti/ep-engine,abhinavdangeti/ep-engine,couchbase/ep-engine,teligent-ru/ep-engine,zbase/ep-engine,couchbase/ep-engine,teligent-ru/ep-engine,owendCB/ep-engine,hisundar/ep-engine,owendCB/ep-engine,couchbaselabs/ep-engine,sriganes/ep-engine,membase/ep-engine,couchbaselabs/ep-engine,jimwwalker/ep-engine,daverigby/kv_engine,owendCB/ep-engine,couchbase/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,membase/ep-engine,jimwwalker/ep-engine,daverigby/kv_engine,couchbaselabs/ep-engine,owendCB/ep-engine,daverigby/ep-engine,hisundar/ep-engine,couchbaselabs/ep-engine