lint: better error handling; count "faultly" files

src/main.rs

@@ -42,6 +42,8 @@ enum FormatterError {
 enum LintError {
     #[error("Lint errors found in {file_count} file(s)")]
     DiagnosticErrors { file_count: usize },
+    #[error("File discovery error: {0}")]
+    FileDiscovery(#[from] anyhow::Error),
 }
 
 #[derive(Parser, Debug, Clone)]