Improve error logging for HTTP error status assertions

[calbach]

There's a bug in most places where the compliance tests expect HTTP errors, as the convention is to blindly dereference a caught exception (may be null, if the server returned a 200), thereby potentially causing an irrelevant stack trace.

For example, ReadGroupSetsGetByNameIT.testSearchForBogusNameeFails incorrectly expects the server to a return a 404 if it doesn't know about the bogus name (in actuality, it should expect an empty list of readGroupSets, see ga4gh/ga4gh-schemas#382). I'll file a separate issue for that, but this was difficult to debug because the test code actually attempts to dereference a null:

testSearchForBogusNameFails Error   N/A

java.lang.NullPointerException
at org.ga4gh.cts.api.reads.ReadGroupSetsGetByNameIT.testSearchForBogusNameeFails(ReadGroupSetsGetByNameIT.java:50)
at org.ga4gh.ctk.AntExecutor.executeAntTask(AntExecutor.java:173)
at org.ga4gh.ctk.TestRunner.doTestRun(TestRunner.java:101))
at org.ga4gh.ctk.TestRunner.doTestRun(TestRunner.java:70)
at org.ga4gh.ctk.Application.run(Application.java:70)
at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:707)
at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:691)
at org.springframework.boot.SpringApplication.afterRefresh(SpringApplication.java:678)
at org.springframework.boot.SpringApplication.doRun(SpringApplication.java:343)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:273)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:971)
at org.springframework.boot.SpringApplication.run(SpringApplication.java:960)
at org.ga4gh.ctk.Application.main(Application.java:53)
at org.springframework.boot.loader.MainMethodRunner.run(MainMethodRunner.java:53)
at java.lang.Thread..run(Thread.java:745)

Ideally, the test would instead print an error which explains what the test expects, and what was actually returned.