terrascan scan fails if path and rego_subdir are not provided together in the toml configfile

[devang-gaur]

• terrascan version: master
• Operating System: mac os

What I Did

╭─dev-gaur@Devangs-MacBook-Pro ~/go/src/github.com/accurics/terrascan ‹master*›
╰─$ rm -rf ~/.terrascan/                                                                                                                                                                                                                                  3 ↵
╭─dev-gaur@Devangs-MacBook-Pro ~/go/src/github.com/accurics/terrascan ‹master*›
╰─$ TERRASCAN_CONFIG=demo/config.toml ./bin/terrascan scan -d pkg/iac-providers/terraform/v12/testdata/deep-modules/modules/m1
2021-03-16T02:52:55.342+0530	error	opa/engine.go:250	error loading rego files{policy path 15 0 /Users/dev-gaur/.terrascan/pkg/policies/opa/rego/aws <nil>} {error 26 0  lstat /Users/dev-gaur/.terrascan/pkg/policies/opa/rego/aws: no such file or directory}
2021-03-16T02:52:55.342+0530	error	runtime/executor.go:121	failed to initialize OPA policy engine
2021-03-16T02:52:55.342+0530	error	cli/run.go:107	scan run failed{error 26 0  failed to initialize OPA policy engine}

configfile.toml looks like this :

[policy]
path = "/Users/dev-gaur/.terra1"
#rego_subdir = "/Users/dev-gaur/.terra2/pkg/policies/opa/rego"

if you comment out either path or rego_subdir... opa engine will error out while trying to compile the rego files.

There should be a check to warn the user to use the use both of these complimentary fields together properly.