Provide more help if lessc is not found.

[ahmadia]

A slightly more useful error message.

I used a ValueError to match the other exceptions, but I think OSError might be a more appropriate exception.

Addresses: #5632

[ahmadia]

And this is minor but a comment in the file indicates lessc@1.5.0 is excluded by the version check, but it isn't.

[minrk]

Do you want to fix the check for 1.5? It should be just changing > to >=.

[ahmadia]

@minrk - I don't understand. distutils.LooseVersion passes this test:

 if V(less_version) > V(max_less_version):

for less_version and max_less_version both equal to 1.5.0, there's an implicit = in there. I can adjust the documentation in the file to match that, unless 1.5.0 is a problem, in which case the logic will need to be fixed.

[minrk]

1.5.0 should be excluded

[ahmadia]

Thanks, I'll figure out what's going on and update the PR.

[ahmadia]

Oh, I'm an idiot. Thanks.

[minrk]

lessc output changes dramatically across versions, so pinning the version helps to avoid massive diffs with small CSS changes. We will probably pin a later version during the 3.0 cycle.

[ahmadia]

@minrk - I redid the commit, this is ready for review again.

[minrk]

thanks