is duplicated by:

• SR-834 Very unhelpful error message when clang++ cannot be invoked

Issue Description:

I just spent ten minutes on Ubuntu 14.0.4 trying to figure out what this error message means:

error: invalid inferred toolchain

Turns out that it's because I hadn't installed `clang`. While this is not a common error message, it is disproportionally likely to affect those people setting up a new `swiftpm` environment, so there is value in fixing it.

It also turns out that this error ("invalid toolchain") can be throw also when `swiftc` isn't found. This really needs to be specific and actionable.