Exit gracefully when chericat cannot attach to a pid

[InterestedInTechAndCake]

The pid could be invalid or it doesn't exist, or chericat (the user who runs it) does not have enough privilege to attach the target process. When this happens, currently chericat would fail an assertion and abort which looks like a crash:

$ bin/chericat -p 12355 -v
Assertion failed: (freep != NULL), function scan_mem, file ./src/mem_scan.c, line 63.
Abort trap (core dumped)

This is way too dramatic, instead chericat should simply exit with a useful message to tell the user why it cannot run.

[InterestedInTechAndCake]

Fix is provided by #12

[InterestedInTechAndCake]

Initial changes merged, there are outstanding minor improvements suggested on #12, hence leaving this issue open until they are fixed.