Fix segfault in disk space check

[micahlee]

Desired Outcome

The desired outcome of this PR is to resolve a segfault that occurs if the user running conjur-preflight doesn't have permission on some of the partition mount points. This manifested with the crash error:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x598e71]

goroutine 7 [running]:
github.com/conjurinc/conjur-preflight/pkg/checks/disk.(*SpaceCheck).Run.func1()
        /conjur-preflight/pkg/checks/disk/space.go:26 +0xf1
created by github.com/conjurinc/conjur-preflight/pkg/checks/disk.(*SpaceCheck).Run
        /conjur-preflight/pkg/checks/disk/space.go:20 +0x6a

Implemented Changes

Previously, an error returned by the usage check was ignored, which led to attempting to read uninitialized memory, leading to the segfault. This error is now properly handled to log it and continue processing.

This PR also fixes all other instances of this class of error. These were not causing issues at the moment, but there are now no unhandled error responses in the project.

Finally, I added the necessary unit tests to include coverage of these scenarios.

Connected Issue/Story

CNJR-830

Definition of Done

At least 1 todo must be completed in the sections below for the PR to be
merged.

Changelog

• The CHANGELOG has been updated, or
• This PR does not include user-facing changes and doesn't require a
  CHANGELOG update

Test coverage

• This PR includes new unit and integration tests to go with the code
  changes, or
• The changes in this PR do not require tests

Documentation

• Docs (e.g. READMEs) were updated in this PR
• A follow-up issue to update official docs has been filed here: [insert issue ID]
• This PR does not require updating any documentation

Behavior

• This PR changes product behavior and has been reviewed by a PO, or
• These changes are part of a larger initiative that will be reviewed later, or
• No behavior was changed with this PR

Security

• Security architect has reviewed the changes in this PR,
• These changes are part of a larger initiative with a separate security review, or
• There are no security aspects to these changes

[telday]

LGTM