Improve error handling in print_files

[gllghr]

ptools still contains some amount of hackathon-quality error handling. In particular, pfiles can panic if a file descriptor in the target process is closed while pfiles is iterating over the entries in /proc/[pid]/fd/. This modifies pfiles to instead print a message to stderr and continue iterating over the file descriptors.

delphix@jg-trunk:~/ptools$ pfiles 17902
17902:  ./test
    0: S_IFCHR mode:620 dev:0,23 ino:4 uid:65433 gid:5 rdev:136,1
       O_RDWR
       /dev/pts/1
    1: S_IFCHR mode:620 dev:0,23 ino:4 uid:65433 gid:5 rdev:136,1
       O_RDWR
       /dev/pts/1
    2: S_IFCHR mode:620 dev:0,23 ino:4 uid:65433 gid:5 rdev:136,1
       O_RDWR
       /dev/pts/1
failed to stat /proc/17902/fd/3: ENOENT: No such file or directory
    4: S_IFSOCK mode:777 dev:0,9 ino:27996 uid:0 gid:0 size:0
       O_RDWR
         SOCK_STREAM
         sockname: AF_INET 0.0.0.0  port: 5005
    ...

I'll follow up with additional PRs to clean up more of the error handling, but this should prevent the issue where pfiles occasionally creates a coredump while a support bundle is being generated.

This also modifies Cargo.toml to ensure that full debug info is generated when we do a release build, since I notice that some debug info was missing while I was investigating this issue.

Closes #9

cc @palashgandhi

[sebroy]

There's a tests directory, would running any tests in there be relevant for this PR?

[gllghr]

ab-pre-push: http://selfservice.jenkins.delphix.com/job/devops-gate/job/master/job/appliance-build-orchestrator-pre-push/3690/