Return a non-zero exit code upon error

This brings us in line with the Unix standard of returning non-zero for a failed command:
https://www.gnu.org/software/bash/manual/html_node/Exit-Status.html

Also, this will allow any callers (like google-java-format-diff.py) to respond appropriately.
[A change](http://r.android.com/2257232) needed to be reverted because `google-java-format-diff.py`
was failing silently, so the author wasn't aware that their change caused a pre-upload hook to stop working.

Fixes #848

COPYBARA_INTEGRATE_REVIEW=#848 from abstractlyZach:return_nonzero 06b2d36
PiperOrigin-RevId: 502399096

Author: abstractlyZach

core/src/main/java/com/google/googlejavaformat/java/Main.java

@@ -79,7 +79,8 @@ static int main(PrintWriter out, PrintWriter err, String... args) {
       return formatter.format(args);
     } catch (UsageException e) {
       err.print(e.getMessage());
-      return 0;
+      // We return exit code 2 to differentiate usage issues from code formatting issues.
+      return 2;
     } finally {
       err.flush();
       out.flush();

core/src/test/java/com/google/googlejavaformat/java/GoogleJavaFormatToolProviderTest.java

@@ -46,7 +46,7 @@ public void testUsageOutputAfterLoadingViaToolName() {
 
     int result = format.run(new PrintWriter(out, true), new PrintWriter(err, true), "--help");
 
-    assertThat(result).isEqualTo(0);
+    assertThat(result).isEqualTo(2);
 
     String usage = err.toString();
 

core/src/test/java/com/google/googlejavaformat/java/MainTest.java

@@ -132,7 +132,7 @@ public void testMain() throws Exception {
     process.waitFor();
     String err = new String(ByteStreams.toByteArray(process.getErrorStream()), UTF_8);
     assertThat(err).contains("Usage: google-java-format");
-    assertThat(process.exitValue()).isEqualTo(0);
+    assertThat(process.exitValue()).isEqualTo(2);
   }
 
   // end to end javadoc formatting test