Error when llvm-tools-preview not installed could be clearer

[AlyoshaVasilieva]

I switched a project of mine from stable Rust to nightly Rust recently, and it was very confusing when later I noticed cargo objcopy wasn't working. The error is:

Failed to execute tool: objcopy
The system cannot find the file specified. (os error 2)

I'd also renamed my project in the meantime, so I got stuck down a tangent trying to figure out how renaming my project broke objcopy, assuming that it wasn't able to find the build output, rather than objcopy itself. It'd be nice if it reminded the user to ensure llvm-tools-preview is present on the current toolchain.