raise exception when the firefox binary isn't actually found, which usually implies the upgrade failed (on windows)

Fixes Issue #6092
Fixes Issue #6847

Author: lukeis

py/selenium/webdriver/firefox/firefox_binary.py

@@ -41,6 +41,9 @@ def __init__(self, firefox_path=None, log_file=None):
         self.command_line = None
         if self._start_cmd is None:
             self._start_cmd = self._get_firefox_start_cmd()
+        if not self._start_cmd.strip():
+          raise Exception("Failed to find firefox binary. You can set it by specifying the path to 'firefox_binary':\n\nfrom selenium.webdriver.firefox.firefox_binary import FirefoxBinary\n\n" +
+            "binary = FirefoxBinary('/path/to/binary')\ndriver = webdriver.Firefox(firefox_binary=binary)")
         # Rather than modifying the environment of the calling Python process
         # copy it and modify as needed.
         self._firefox_env = os.environ.copy()