Fix silent error swallowing in cmd/main.go

Handle the error returned by os.ReadFile() in insert_purls instead
of discarding it with _. Previously, if a data file could not be read,
zero PURLs were silently inserted into the FST with no indication of
failure.

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Co-Authored-By: Codebuff <noreply@codebuff.com>

Author: krisharma955

cmd/main.go

@@ -55,9 +55,11 @@ func main() {
 }
 
 func insert_purls(builder *vellum.Builder, file string) int {
-	var err error
 	// #nosec G304
-	data, _ := os.ReadFile(file)
+	data, err := os.ReadFile(file)
+	if err != nil {
+		log.Fatal(err)
+	}
 	lines := strings.FieldsFunc(string(data), func(r rune) bool {
 		return r == '\n'
 	})