update rtdev:check_node/1 to work w/ atom version specifiers

previous version specifiers were strings, the change to atoms (e.g. `previous`, `legacy`)
caused an error in `rtdev:check_node/1`. Also, updated message to suggest that configuration could be the problem.

Example of the error on master:
```
22:44:13.245 [warning] verify_basic_upgrade failed: {badarg,[{erlang,'++',[previous," installed"],[]},{rtdev,check_node,1,[{file,"src/rtdev.erl"},{line,435}]},{rtdev,'-deploy_nodes/1-lc$^1/1-1-',1,[{file,"src/rtdev.erl"},{line,223}]},{rtdev,deploy_nodes,1,[{file,"src/rtdev.erl"},{line,223}]},{rt,deploy_nodes,2,[{file,"src/rt.erl"},{line,272}]},{rt,build_cluster,3,[{file,"src/rt.erl"},{line,767}]},{verify_basic_upgrade,confirm,0,[{file,"tests/verify_basic_upgrade.erl"},{line,29}]}]}
```