bblfshctl driver install --force is misleading

[carlosms]

The bblfshctl driver install --force is misleading, it gives you the understanding that you are forcing to install a driver even if it is already installed. But what is actually happening is that the error message becomes a warning.

$ docker exec -it lookout-demo_bblfsh_1 bblfshctl driver list|grep javascript
| javascript | docker://bblfsh/javascript-driver:latest | v2.5.0      | beta   | 2 months | alpine | 1.10-alpine | node:8-alpine        |

$ docker exec -it lookout-demo_bblfsh_1 bblfshctl driver install javascript bblfsh/javascript-driver:v1.2.0
Installing javascript language driver from "bblfsh/javascript-driver:v1.2.0"... driver already installed: javascript (image reference: docker://bblfsh/javascript-driver:v1.2.0)

$ docker exec -it lookout-demo_bblfsh_1 bblfshctl driver install --force javascript bblfsh/javascript-driver:v1.2.0
Installing javascript language driver from "bblfsh/javascript-driver:v1.2.0"... Warning: driver already installed: javascript (image reference: docker://bblfsh/javascript-driver:v1.2.0)

$ docker exec -it lookout-demo_bblfsh_1 bblfshctl driver list|grep javascript
| javascript | docker://bblfsh/javascript-driver:latest | v2.5.0      | beta   | 2 months | alpine | 1.10-alpine | node:8-alpine        |

[bzz]

Related to #184 so most probably can be addressed together.