[java] JavaCharStream throws an Error on invalid escape

[oowekyala]

Affects PMD Version: 6.23.0

Description: Javacc throws an Error on invalid escapes, which isn't caught, so it crashes the whole program. It also doesn't mention the filename.

Exception Stacktrace:

Exception in thread "main" java.lang.Error: Invalid escape character at line 1 column 2.
	at net.sourceforge.pmd.lang.ast.JavaCharStream.readChar(JavaCharStream.java:331)
	at net.sourceforge.pmd.lang.ast.JavaCharStream.BeginToken(JavaCharStream.java:193)
	at net.sourceforge.pmd.lang.java.ast.JavaParserTokenManager.getNextToken(JavaParserTokenManager.java:2271)
	at net.sourceforge.pmd.lang.java.ast.JavaParser.<init>(JavaParser.java:12555)
	at net.sourceforge.pmd.lang.java.AbstractJavaParser.createJavaParser(AbstractJavaParser.java:46)
	at net.sourceforge.pmd.lang.java.JavaLanguageParser.createJavaParser(JavaLanguageParser.java:41)
	at net.sourceforge.pmd.lang.java.AbstractJavaParser.parse(AbstractJavaParser.java:62)
	at net.sourceforge.pmd.SourceCodeProcessor.parse(SourceCodeProcessor.java:121)
	at net.sourceforge.pmd.SourceCodeProcessor.processSource(SourceCodeProcessor.java:185)
	at net.sourceforge.pmd.SourceCodeProcessor.processSourceCodeWithoutCache(SourceCodeProcessor.java:107)
	at net.sourceforge.pmd.SourceCodeProcessor.processSourceCode(SourceCodeProcessor.java:89)
	at net.sourceforge.pmd.SourceCodeProcessor.processSourceCode(SourceCodeProcessor.java:51)
	at net.sourceforge.pmd.processor.PmdRunnable.call(PmdRunnable.java:78)
	at net.sourceforge.pmd.processor.PmdRunnable.call(PmdRunnable.java:24)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)

Code Sample demonstrating the issue:

\u00k0 // not a valid unicode escape because k is not a valid hex digit 

Steps to reproduce:

1. analyse a file with an invalid escape like above

Running PMD through: any