Use str instead of repr for exception msg

Another fix for exception message formatting.

Before:
```
WebDriverException: Message: u"disconnected: Unable to receive message from renderer\n  (Session info: chrome=38.0.2125.104)\n  (Driver info: chromedriver=2.11.298611 (d1120fdf51badec2f7b63a96e19a58d4783de84d),platform=Mac OS X 10.10.0 x86_64) (WARNING: The server did not provide any stacktrace information)\nCommand duration or timeout: 504 milliseconds\nBuild info: version: '2.43.1', revision: '5163bce', time: '2014-09-10 16:27:33'\nSystem info: host: 'mbp.local', ip: '10.37.129.2', os.name: 'Mac OS X', os.arch: 'x86_64', os.version: '10.10', java.version: '1.7.0_55'\nSession ID: 6eb12a8df9b4e0841d298413114b6f2a\nDriver info: org.openqa.selenium.chrome.ChromeDriver\nCapabilities [{platform=MAC, acceptSslCerts=true, javascriptEnabled=true, browserName=chrome, chrome={userDataDir=/var/folders/59/335f0jxs7sg1rx0c9dzjjdzc0000gp/T/.org.chromium.Chromium.uBGbMS}, rotatable=false, locationContextEnabled=true, mobileEmulationEnabled=false, version=38.0.2125.104, takesHeapSnapshot=true, cssSelectorsEnabled=true, databaseEnabled=false, handlesAlerts=true, browserConnectionEnabled=false, nativeEvents=true, webStorageEnabled=true, applicationCacheEnabled=false, takesScreenshot=true}]"
```
After:
```
WebDriverException: Message: u"disconnected: Unable to receive message from renderer
  (Session info: chrome=38.0.2125.104)
  (Driver info: chromedriver=2.11.298611 (d1120fdf51badec2f7b63a96e19a58d4783de84d),platform=Mac OS X 10.10.0 x86_64) (WARNING: The server did not provide any stacktrace information)
Command duration or timeout: 504 milliseconds
Build info: version: '2.43.1', revision: '5163bce', time: '2014-09-10 16:27:33'
System info: host: 'mbp.local', ip: '10.37.129.2', os.name: 'Mac OS X', os.arch: 'x86_64', os.version: '10.10', java.version: '1.7.0_55'
Session ID: 6eb12a8df9b4e0841d298413114b6f2a
Driver info: org.openqa.selenium.chrome.ChromeDriver
Capabilities [{platform=MAC, acceptSslCerts=true, javascriptEnabled=true, browserName=chrome, chrome={userDataDir=/var/folders/59/335f0jxs7sg1rx0c9dzjjdzc0000gp/T/.org.chromium.Chromium.uBGbMS}, rotatable=false, locationContextEnabled=true, mobileEmulationEnabled=false, version=38.0.2125.104, takesHeapSnapshot=true, cssSelectorsEnabled=true, databaseEnabled=false, handlesAlerts=true, browserConnectionEnabled=false, nativeEvents=true, webStorageEnabled=true, applicationCacheEnabled=false, takesScreenshot=true}]"
```
Use 

```
exception_msg += "\nStacktrace: %s" % "\n".join(self.stacktrace)