Map config deserilaization errors to CommandOutcome.

[andrus]

Mistakenly using a String where an array is expected. The result is stack trace with a real cause, and a very generic unhelpful command error:

Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.util.ArrayList out of VALUE_STRING token
 at [Source: N/A; line: -1, column: -1] (through reference chain: org.objectstyle.taskextractor.TaskExtractorFactory["extractors"]->java.util.ArrayList[0]->org.objectstyle.taskextractor.git.GitExtractorFactory["repositories"])
	at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:148)
... 
com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2005)
	at io.bootique.config.jackson.JsonNodeConfigurationFactory.config(JsonNodeConfigurationFactory.java:38)
	... 24 more
Error running command '-m 2017-03 -c andrus.yml': Command exception: 'Error creating config'.