Improve turbo prune error messaging when a scope is invalid

[rmolinamir]

Describe the feature you'd like to request

Currently, turbo prune outputs the following error whenever a scope is invalid (e.g. it doesn't match any of the package.jsons names):

$ RUN turbo prune --scope=web --docker:
9 0.418 panic: runtime error: invalid memory address or nil pointer dereference
9 0.418 [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x7d4121]

Describe the solution you'd like

Better error messaging, indicating the invalid scope when using turbo prune.

Describe alternatives you've considered

N/A