If shaded client artifacts are built without the -Prelease flag, make sure they'll fail loudly if used

If someone builds the shaded jars and doesn't pass the -Prelease flag, we get near-empty jars.

We should make sure that if they're loaded they fail loudly rather than as no-ops as they are now.

On a Hadoop 3 cluster without YARN-7190, this results in the following confusing result:

Busbey-MBA:hbase busbey$ ./hbase-assembly/target/hbase-2.1.0-SNAPSHOT-client/bin/hbase version
HBase 1.2.6
Source code repository file:///home/busbey/projects/hbase/hbase-assembly/target/hbase-1.2.6 revision=Unknown
Compiled by busbey on Mon May 29 02:25:32 CDT 2017
From source with checksum 7e8ce83a648e252758e9dae1fbe779c9

On a cluster not impacted by YARN-7190 we'll jsut get a confusing class not found exception.