[circt] Improve error message on firtool not found

Improve the error message that trips if firtool is not found.  Add a new
exception and throw this when execution of the firtool process fails.
This commit tries to narrowly only match on the exact condition that
this failed due to firtool not being installed.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>