Improve panic handling

[vinc]

I found that I could improve the panic handlers while working on error messages for the brainfuck interpreter in #750

[vinc]

Panic in userspace:

~
> hello panic
Error: Panicked

[vinc]

Panic inside the kernel:

~
> panic kernel panic
Error: Panicked at src/usr/shell.rs:567:23
       ^^^^^^^^ kernel panic

[vinc]

Trying to execute a tiny file (less than 4 bytes) would previously crash the kernel:

~
> print no => /tmp/no

~
> /tmp/no
Error: Panicked at src/sys/process.rs:363:15
       ^^^^^^^^ range end index 4 out of range for slice of length 3

Now the kernel just returns an error:

~
> /tmp/no
Error: Could not execute '/tmp/no'

[vinc]

Screenshot: