When /proc isn't mounted lean aborts: please catch exceptions and print messages from them

[yurivict]

When /proc isn't mounted lean aborts:

$ lean 
Abort trap

You have a throw statement when file open fails, but it looks that this exception isn't caught.

[gebner]

Access to /proc is required because lean needs to determine the absolute path of the executable (in order to find the core library path). The error message should be better though, I agree.

Is there a reason you want to run lean on a system without /proc?

[yurivict]

Is there a reason you want to run lean on a system without /proc?

On FreeBSD /proc isn't enabled by default and people would run without it.