feat: better exception handling

[brianreichtcs]

If approved this PR:

1. Adds a dependency on psr/log
2. Adds a private method logFailedRequest to Client which, if a logger is present, logs the failed request/response.
3. Adds a call to logFailedRequest() everywhere a SmarterUException is thrown.

With this change in place, we can actually capture information we need to troubleshoot integration issues.

One thing you might wonder about is, why am I not testing that the logger gets data when a failure occurs? The reason is because psr/log doesn't ship a test logger, only NullLogger, which is a "no-op" implementation that I cannot expect to validate log events.

[brianreichtcs]

Closing this until I'm done making changes

[tomegantcs]

I don't think it would be difficult to add tests to see that something was sent to a logger. In tests/Client/AddUsersToGroupClientTest.php for instance line 287 would be changed to:

$logger = $this->createMock(\Psr\Log\LoggerInterface::class);
$logger->expects($this->once())->method('error')->with(
    $this->identicalTo('Failed to make request to SmarterU API. See context for request/response details.'),
    $this->identicalTo([
        'request' => // I'm not sure but maybe you put 'cat' here and see what the failure looks like to get the text?
        'response' => $body
    ])
);
$client
    ->setLogger($logger)
    ->setHttpClient($httpClient);