embulk-input-redshft: should throw ConfigException if the database doesn't exist

It should be wrapped and thrown as ConfigException.

```
java.lang.RuntimeException: org.postgresql.util.PSQLException: FATAL: database "xxx mydb xxx" does not exist
at com.google.common.base.Throwables.propagate(Throwables.java:160) ~[guava-18.0.jar:na]
at org.embulk.input.jdbc.AbstractJdbcInputPlugin.transaction(AbstractJdbcInputPlugin.java:144) ~[na:na]
at org.embulk.plugin.compat.InputPluginWrapper.transaction(InputPluginWrapper.java:57) ~[embulk-core-0.7.7.jar:na]
at org.embulk.exec.BulkLoader.doRun(BulkLoader.java:517) ~[embulk-core-0.7.7.jar:na]
at org.embulk.exec.BulkLoader.access$100(BulkLoader.java:33) ~[embulk-core-0.7.7.jar:na]
at org.embulk.exec.BulkLoader$1.run(BulkLoader.java:379) ~[embulk-core-0.7.7.jar:na]
at org.embulk.exec.BulkLoader$1.run(BulkLoader.java:375) ~[embulk-core-0.7.7.jar:na]
at org.embulk.spi.Exec.doWith(Exec.java:25) ~[embulk-core-0.7.7.jar:na]
at org.embulk.exec.BulkLoader.run(BulkLoader.java:375) ~[embulk-core-0.7.7.jar:na]
... ...
Caused by: org.postgresql.util.PSQLException: FATAL: database "my_database" does not exist
at org.postgresql.Driver$ConnectThread.getResult(Driver.java:365) ~[postgresql-9.3-1102-jdbc4.jar:na]
at org.postgresql.Driver.connect(Driver.java:288) ~[postgresql-9.3-1102-jdbc4.jar:na]
at org.embulk.input.RedshiftInputPlugin.newConnection(RedshiftInputPlugin.java:81) ~[na:na]
at org.embulk.input.RedshiftInputPlugin.newConnection(RedshiftInputPlugin.java:14) ~[na:na]
at org.embulk.input.jdbc.AbstractJdbcInputPlugin.transaction(AbstractJdbcInputPlugin.java:141) ~[na:na]
... 23 common frames omitted
```