Fix #38, Handle lseek error.

[zanzaben]

Describe the contribution
Fixes #38
Checks if lseek returns an error and reports it.

Fixes #40
Use exit(1)

Testing performed
Build and run unit test

Expected behavior changes
No impact to behavior

System(s) tested on
Ubuntu 20.04

Contributor Info - All information REQUIRED for consideration of pull request
Alex Campbell GSFC

[astrogeco]

CCB 2021-03-03 APPROVED with CHANGES

• add exit 1 instead since exit 0 means success, open new issue
• Conversation about using strerror
• Does it make sense to check against the desired offset instead?
• Use off_t instead of int when checking the output

[jphickey]

Requesting a couple more fixups, otherwise good. See comments inline.

[jphickey]

Sorry missed something in my previous skim-through. Hopefully this is the last nitpick!

[jphickey]

strerror() takes an errno, not the return code (the latter is -1 by definition).

So it should be strerror(errno) ... OR just use perror() routine instead. That just simplifies when you want to print the current errno.

We should also do this EVERY time we do exit(1) - not just here.

I do recommend using this function when all you want to do is print the error in the simplest way possible. It's quick and easy. See: https://man7.org/linux/man-pages/man3/perror.3p.html

[zanzaben]

Moving this to #42