Give better error message when relative path is given to sstableloader

[asias]

Relative path does not work. The error message is confusing.

sstableloader -d 127.0.0.1 1631753839123
java.lang.NullPointerException
        at com.scylladb.tools.BulkLoader.main(BulkLoader.java:1341)

Full path worsk:

 sstableloader -v -d 127.0.0.1 `pwd`/1631753839123
===== Using optimized driver!!! =====
Adding sstable /home/asias/src/cloudius-systems/scylla/mycluster/tmp/1/data/keyspace1/standard1-17b27b0015cb11ec860b9c2b16e96e04/md-7-big
Adding sstable /home/asias/src/cloudius-systems/scylla/mycluster/tmp/1/data/keyspace1/standard1-17b27b0015cb11ec860b9c2b16e96e04/md-6-big
ERROR 09:00:41,673 LEAK DETECTED: a reference (org.apache.cassandra.utils.concurrent.Ref$State@4a9ed687) to class org.apache.cassandra.io.sstable.format.SSTableReader$InstanceTidier@2000064150:/home/asias/src/cloudius-systems/scylla/mycluster/tmp/1/data/keyspace1/standard1-17b27b0015cb11ec860b9c2b16e96e04/md-7-big was not released before the reference was garbage collected
 99% done.  1577240 statements sent (in   788661 batches,        0 failed).
 1577248 statements generated.
 1577248 cql rows processed in   788624 partitions.
       0 cql rows and        0 partitions deleted.
       0 local and        0 remote counter shards where skipped