Compiler does not warn about invalid check names when using -XepPatchChecks

### What version of Error Prone are you using?

2.3.1

### Does this issue reproduce with the latest release?

Yes

### What did you do?

I ran Errorprone and passed an invalid check name to the `-XepPatchChecks` option.

```bash
$ echo 'public class Foo {}' > Foo.java
$ java -Xbootclasspath/p:error_prone_ant-2.3.1.jar \
com.google.errorprone.ErrorProneCompiler \
-XepPatchChecks:NotAValidCheck \
-XepPatchLocation:`pwd` \
Foo.java
```

### What did you expect to see?

I expected the compiler to report an error, since an invalid check name was used and the `-XepIgnoreUnknownCheckNames` flag was not provided.

### What did you see instead?

No error was reported. The Errorprone compiler ran successfully on the file, apparently ignoring the invalid check.

Note that the compiler does report an error, as expected, when passing a flag like `-Xep:NotAValidCheck`. However, it doesn't report an error when passing a flag like `-XepPatchChecks:NotAValidCheck`.