Improve error message when trying to call project on a symlink.

[valadez-abel]

Issue

When trying to call project on a symlink, you currently get an error as follows:

BUILD FAILED
The path `usr/bin/ar` cannot be projected in the artifact ``(toolchains//xxx#92a9fd4d9d0db33d))/toolchain`, action: (target: `toolchains//xxx#92a9fd4d9d0db33d)`, id: `0`)`

Caused by:
    Find would traverse a leaf at path: `buck-out/v2/gen/toolchains/xxx/__toolchain__/usr`

The behavior isn't supported, but the error isn't super helpful, even after it was improved with #176

Proposal

Add something along the lines of Are you trying to call project on a symlink? to the error message, which is more user actionable.

[alexlian]

Per #907 landing, closing.