Display error message correctly when install/removeApp fails on iOS.

When the install/remove commands fail, this is what I get in Appium:
```
info: Installing app using cmd: fruitstrap install --id bar --bundle foo.app
info: Cleaning up appium session
error: Failed to start an Appium session, err was: Error: Command failed: 
info: Responding to client with error: {"status":33,"value":{"message":"A new session could not be created. (Original error: Command failed: )","killed":false,"code":1,"signal":null,"origValue":"Command failed: "},"sessionId":null}
```
This is a fix for displaying the custom message (as intended) so that I can get:
```
info: Installing app using cmd: fruitstrap install --id bar --bundle foo.app
info: Cleaning up appium session
error: Failed to start an Appium session, err was: Error: Unable to install [foo.app] to device with id [bar]. Error [Error: Command failed: ]
info: Responding to client with error: {"status":33,"value":{"message":"A new session could not be created. (Original error: Unable to install [foo.app] to device with id [bar]. Error [Error: Command failed: ])","origValue":"Unable to install [foo.app] to device with id [bar]. Error [Error: Command failed: ]"},"sessionId":null}
```
Not sure if it's the best way to handle this, but 'Error: Command failed:' is a bit suboptimal.