Improve error/check for Chapel installation

[bradcray]

In this StackOverflow post, a user was confused about why pip installing Arkouda was leading to an error about Chapel not being installed when, to him, it clearly was. This makes me wonder whether:

(a) Arkouda could be checking for Chapel installation via which chpl rather than $CHPL_HOME (in general, $CHPL_HOME is not required to be set when using chpl); (b) if $CHPL_HOME must be set for some reason, updating the error message to indicate that the problem is actually that $CHPL_HOME isn't set (though this worries me a little as homebrew users, for example, don't tend to have $CHPL_HOME set.