PoolMiner - show better error when pool_host not set

[fireduck64]

INFO: Not loading 9 teapot.9 (hippo), directory not present: /data/teapot/teapot.9
Jan 14, 2021 10:56:22 AM snowblossom.miner.FieldScan scan
INFO: Not loading 10 teapot.10 (shai-hulud), directory not present: /data/teapot/teapot.10
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
Exception in thread "main" java.lang.NullPointerException: target
at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:910)
at io.grpc.internal.AbstractManagedChannelImplBuilder.(AbstractManagedChannelImplBuilder.java:194)
at io.grpc.netty.NettyChannelBuilder.(NettyChannelBuilder.java:136)
at io.grpc.netty.NettyChannelBuilder.(NettyChannelBuilder.java:131)
at io.grpc.netty.NettyChannelBuilder.forAddress(NettyChannelBuilder.java:117)
at io.grpc.netty.NettyChannelProvider.builderForAddress(NettyChannelProvider.java:37)
at io.grpc.netty.NettyChannelProvider.builderForAddress(NettyChannelProvider.java:23)
at io.grpc.ManagedChannelBuilder.forAddress(ManagedChannelBuilder.java:39)
at snowblossom.miner.PoolClient.subscribe(PoolClient.java:74)
at snowblossom.miner.PoolMiner.(PoolMiner.java:99)
at snowblossom.miner.PoolMiner.main(PoolMiner.java:42)