Safely Account for a YARA Compilation Failure

[phutelmyer]

Describe the change
As identified in #410, Strelka will fail to load YARA, and subsequently fail to scan, if an unsupported YARA rule is ingested. This can be fixed by curating YARA rules prior to use in Strelka, however, to account for safe loading, we are now checking for a properly loaded compiled YARA prior to use.

Note: This may still fail if users remove the default test YARA and push unsupported YARA rules.

Describe testing procedures
A test using an unsupported YARA rule was created. This test successfully passes, which prior to the changes, did not. Strelka should not fail anymore using the default patterns + any unsupported patterns.

Sample output
N/A

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of and tested my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[phutelmyer]

Note
As Strelka supports pre-compiled rulesets, we recommend using those to prevent rule compilation failures.
Compiling rules in a pipeline ahead of Strelka ensures that all expected rules will load properly every time.