bin/lessc: Make sure path.dirname gets passed strings

[addaleax]

Since nodejs/node@08085c49b6e, which will be part of Node.js v6.0, functions from the path core module (like dirname) will require the input to be a string.

Because .sourceMapFullFilename and .sourceMapInputFilename in bin/lessc may be undefined, default to passing '.' to path.dirname instead. (In Node ≤ v5.x, path.dirname(undefined) === '.'.)

Disclosure: I basically don’t know anything about less internals. What I know is that this fixes my build; there’s a good chance that there is a more appropriate solution for this problem. I also have no idea where a test for this particular problem would go, but since this issue can be reproduced by simply running lessc <input>, I don’t know if that’s even necessary.

Also, this line suppresses all uncaught exceptions, which made debugging kinda hard. Again, I don’t know whether that’s the best way for less, but I’d suggest adding an explicit handler via process.on('uncaughtException', …).

[seven-phases-max]

Merging (I guess this can't hurt at least... Though I'm in doubts if it should be explicit cwd string instead of . (which probably could have some weird side-effects in edge cases).

[seven-phases-max]

Thanks!