Fix panic when the expression includes invalid references

[wata727]

If an expression includes invalid references, panic and exit instead of returning an error. The following configuration is an example:

resource "aws_s3_bucket" "example" {
  bucket = "example"
  acl    = "private"
  region = invalid // invalid reference
}

Before

Panic: Invalid reference: A reference to a resource type must be followed by at least one attribute access, specifying the resource name.
 -> 0: main.main.func1: /main.go(30)
 -> 1: runtime.gopanic: /panic.go(679)
 -> 2: github.com/wata727/tflint/tflint.isEvaluableExpr: /runner.go(677)
 -> 3: github.com/wata727/tflint/tflint.(*Runner).EvaluateExpr: /runner.go(207)
 -> 4: github.com/wata727/tflint/rules/awsrules.(*AwsS3BucketInvalidRegionRule).Check.func1: /aws_s3_bucket_invalid_region.go(75)
 -> 5: github.com/wata727/tflint/tflint.(*Runner).WalkResourceAttributes: /runner.go(471)
 -> 6: github.com/wata727/tflint/rules/awsrules.(*AwsS3BucketInvalidRegionRule).Check: /aws_s3_bucket_invalid_region.go(73)
 -> 7: github.com/wata727/tflint/cmd.(*CLI).Run: /cli.go(152)
 -> 8: main.main: /main.go(43)
 -> 9: runtime.main: /proc.go(203)
 -> 10: runtime.goexit: /asm_amd64.s(1357)

TFLint crashed... :(
Please attach an output log, describe the situation and version that occurred and post an issue to https://github.com/wata727/tflint/issues

After

Failed to check `aws_s3_bucket_invalid_region` rule. An error occurred:

Error: Failed to parse an expression in s3.tf:4; Invalid reference: A reference to a resource type must be followed by at least one attribute access, specifying the reso
urce name.

[wata727]

[Golint] error strings should not be capitalized or end with punctuation or a newline (link)

Since this error is returned from Terraform, there is nothing we can do here.