Nicer ApiErrorException::__toString()

[richardm-stripe]

Fixes #1561

Changes ApiErrorException::__toString() to have more useful information (a stack trace). This should be a safe change: __toString() is for human consumption and not intended to be consumed programatically.

Before this change:

(Status 200) (Request req_test) message

After this change:

Error sending request to Stripe: (Status 200) (Request req_test) message
Mock_ApiErrorException_21f57d2b: message in /Users/richardm/stripe/stripe-php/lib/Exception/ApiErrorException.php:38
Stack trace:
#0 /Users/richardm/stripe/stripe-php/tests/Stripe/Exception/ApiErrorExceptionTest.php(17): Stripe\Exception\ApiErrorException::factory('message', 200, '{"error": {"cod...', Array, Array, 'some_code')
#1 /Users/richardm/stripe/stripe-php/tests/Stripe/Exception/ApiErrorExceptionTest.php(45): Stripe\Exception\ApiErrorExceptionTest->createFixture()
#2 /Users/richardm/stripe/stripe-php/vendor/phpunit/phpunit/src/Framework/TestCase.php(1608): Stripe\Exception\ApiErrorExceptionTest->testToString()
#3 /Users/richardm/stripe/stripe-php/vendor/phpunit/phpunit/src/Framework/TestCase.php(1214): PHPUnit\Framework\TestCase->runTest()
#4 /Users/richardm/stripe/stripe-php/vendor/phpunit/phpunit/src/Framework/TestResult.php(728): PHPUnit\Framework\TestCase->runBare()
#5 /Users/richardm/stripe/stripe-php/vendor/phpunit/phpunit/src/Framework/TestCase.php(964): PHPUnit\Framework\TestResult->run(Object(Stripe\Exception\ApiErrorExceptionTest))
#6 /Users/richardm/stripe/stripe-php/vendor/phpunit/phpunit/src/Framework/TestSuite.php(684): PHPUnit\Framework\TestCase->run(Object(PHPUnit\Framework\TestResult))
#7 /Users/richardm/stripe/stripe-php/vendor/phpunit/phpunit/src/TextUI/TestRunner.php(653): PHPUnit\Framework\TestSuite->run(Object(PHPUnit\Framework\TestResult))
#8 /Users/richardm/stripe/stripe-php/vendor/phpunit/phpunit/src/TextUI/Command.php(144): PHPUnit\TextUI\TestRunner->run(Object(PHPUnit\Framework\TestSuite), Array, Array, true)
#9 /Users/richardm/stripe/stripe-php/vendor/phpunit/phpunit/src/TextUI/Command.php(97): PHPUnit\TextUI\Command->run(Array, true)
#10 /Users/richardm/stripe/stripe-php/vendor/phpunit/phpunit/phpunit(98): PHPUnit\TextUI\Command::main()
#11 /Users/richardm/stripe/stripe-php/vendor/bin/phpunit(123): include('/Users/richardm...')
#12 {main}

[richardm-stripe]

Released in https://github.com/stripe/stripe-php/releases/tag/v12.1.0