Steward should fail or warn if not-existing browser selected

[MartinKristof]

If you make a typo in browser name, steward does not warn you and attempts to start with given (not existing) browser, what leads to weird error from selenium webdriver (it obviously tries to start some random/default browser, Opera in my case).

./steward.php run-tests dev3 thisisnotabrowser
Steward 1.0.0 is running the tests... Just for you <3!
Browser: thisisnotabrowser
Environment: dev3
Selenium server (hub) url: http://localhost:4444, trying connection...OK

...

[2015-05-11 11:38:32]: Initializing "thisisnotabrowser" WebDriver for "Lmc\Jobs\Elite\FooBarTest::foo"
PHP Fatal error:  Uncaught exception 'UnknownServerException' with message 'Unable to find executable for product Opera Desktop' in /media/martin/vagrant-project/Project/vagrant-lmc/repos/vpv.lmc.cz/selenium-tests/vendor/facebook/webdriver/lib/WebDriverExceptions.php:79
Stack trace:
#0 /media/martin/vagrant-project/Project/vagrant-lmc/repos/vpv.lmc.cz/selenium-tests/vendor/facebook/webdriver/lib/remote/HttpCommandExecutor.php(268): WebDriverException::throwException(13, 'Unable to find ...', Array)
#1 /media/martin/vagrant-project/Project/vagrant-lmc/repos/vpv.lmc.cz/selenium-tests/vendor/facebook/webdriver/lib/remote/RemoteWebDriver.php(81): HttpCommandExecutor->execute(Object(WebDriverCommand))
#2 /media/martin/vagrant-project/Project/vagrant-lmc/repos/vpv.lmc.cz/selenium-tests/vendor/lmc/steward/src/Listener/WebdriverListener.php(128): RemoteWebDriver::create('http://localhos...', Object(DesiredCapabilities), 120000, 3600000)
#3 /media/martin/vagrant-project/Project/vagrant-lmc/repos/vpv.lmc.cz/selenium-tests/vendor/lmc/ste in /media/martin/vagrant-project/Project/vagrant-lmc/repos/vpv.lmc.cz/selenium-tests/vendor/facebook/webdriver/lib/WebDriverExceptions.php on line 79