SUMO-15772 Sumo client should correctly provide HTTP Source endpoint

Also, show a nicer error message when service is unavailable or when the service responds with a non-JSON error message.

Before:

```
com.sumologic.client.SumoClientException: Runtime error reading server response
    at com.sumologic.client.util.HttpUtils.doRequest(HttpUtils.java:225)
    at com.sumologic.client.util.HttpUtils.doRequest(HttpUtils.java:173)
    at com.sumologic.client.util.HttpUtils.post(HttpUtils.java:79)
    at com.sumologic.client.collectors.CollectorsClient.create(CollectorsClient.java:37)
    at com.sumologic.client.SumoLogicClient.createCollector(SumoLogicClient.java:236)
    at com.sumologic.SourceCreate.main(SourceCreate.java:31)
    ... 6 more
Caused by: com.sumologic.client.SumoClientException: Exception while parsing JSON: 

    at com.sumologic.client.SumoServerException.<init>(SumoServerException.java:47)
    at com.sumologic.client.util.HttpUtils.doRequest(HttpUtils.java:211)
    ... 11 more
Caused by: com.fasterxml.jackson.databind.JsonMappingException: No content to map due to end-of-input
 at [Source: java.io.StringReader@6b4d6015; line: 1, column: 1]
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:163)
    at com.fasterxml.jackson.databind.ObjectMapper._initForReading(ObjectMapper.java:2623)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2565)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:1817)
    at com.sumologic.client.SumoServerException.<init>(SumoServerException.java:38)
    ... 12 more
```

Now:

```
com.sumologic.client.SumoServerException: The server responded with HTTP Status: 503
	at com.sumologic.client.util.HttpUtils.doRequest(HttpUtils.java:215)
	at com.sumologic.client.util.HttpUtils.doRequest(HttpUtils.java:173)
	at com.sumologic.client.util.HttpUtils.post(HttpUtils.java:79)
	at com.sumologic.client.collectors.CollectorsClient.create(CollectorsClient.java:37)
	at com.sumologic.client.SumoLogicClient.createCollector(SumoLogicClient.java:236)
	at com.sumologic.SourceCreate.main(SourceCreate.java:31)
	... 6 more

```