fix: missing tsconfig error shouldn't say undefined (#383)

- per the conditional above this line, `file` is falsey, so printing it doesn't make sense
  - per same conditional though, `pluginOptions.tsconfig` exists, so we can print that

- fixes a test TODO/FIXME that had to workaround this bug as well

__tests__/integration/errors.spec.ts

@@ -34,7 +34,7 @@ test("integration - tsconfig errors", async () => {
   // TODO: move to parse-tsconfig unit tests?
   expect(genBundle("semantic.ts", {
     tsconfig: "non-existent-tsconfig",
-  })).rejects.toThrow("rpt2: failed to open 'undefined'"); // FIXME: bug: this should be "non-existent-tsconfig", not "undefined"
+  })).rejects.toThrow("rpt2: failed to open 'non-existent-tsconfig'");
 });
 
 test("integration - semantic error", async () => {

src/parse-tsconfig.ts

@@ -15,7 +15,7 @@ export function parseTsConfig(context: IContext, pluginOptions: IOptions)
 
 	// if the value was provided, but no file, fail hard
 	if (pluginOptions.tsconfig !== undefined && !fileName)
-		throw new Error(`rpt2: failed to open '${fileName}'`);
+		throw new Error(`rpt2: failed to open '${pluginOptions.tsconfig}'`);
 
 	let loadedConfig: any = {};
 	let baseDir = pluginOptions.cwd;