Add a check for existence of `android` command in grunt-helpers.js.

I had an incorrectly-set `ANDROID_HOME` env variable, and got a very cryptic error message when running `reset.sh --dev --android` - the same error message reported in #1064.

This pull request verifies the existence of the necessary `android` command-line tool and reports a clearer error message.

Side note: expanded the error messages returned in adb.js a little bit.