Fix null pointer exception when the remote session can not be started

When appium can not start a new session (wrong capabilities, ...), the java client thrown this exception:
_java.lang.NullPointerException at io.appium.java_client.AppiumDriver.execute(AppiumDriver.java:78)_

If errorHandler is static, it is initialized before constructor and the original error will be returned:
_org.openqa.selenium.WebDriverException: Error forwarding the new session cannot find : Capabilities [{automationName=Appium, platform=MAC, platformName=iOS, browserName=safari}]_