Better handling of the case where vg is not installed

[adamnovak]

Some people are having trouble with the tube map not being able to find vg. In this case, we bombard them with useless exited with code -2 errors in the browser, and cryptic errors about ENOENT in the console.

We should anticipate the case where vg is not installed and provide a genuinely useful error message that instructs the user about how to install vg and add it to their PATH.

We should improve the install documentation to guide users through doing this properly the first time.

We could attempt, on supported platforms, to ourselves download a copy of vg if it appears to be missing.