Exclusion file: tell unused exclusion elements in the file

Linkage Checker tells unused exclusion elements that did not match any linkage errors. This shows improvement in the dependency compatibility. The exit code is 1 to indicate the exclusion file needs to be updated accordingly.

# What the output looks like?

```
LinkageCheckerMain -a foo:bar:1.2.3 -e exclusions.xml

...

The following rule did not match any linkage errors:

exclusions.xml: line 56:








GraalVM-related libraries depend on Java Compiler Interface (JVMCI) that
only exists in special JDK. These missing classes are false positives, because
the code is only invoked when running in a GraalVM.
https://github.com/GoogleCloudPlatform/cloud-opensource-java/issues/929


```