Improve exception readability in case a directory can't be deleted be…

…cause it still contains files

Use suppressed exceptions rather than wrap the exception so that we get
the primary cause first, then details.

Also resolve files relative to their containing folders.

Before:

```
java.io.IOException: [REDACTED]/target/tmp/j h16565971146678333138/users/Fred_10744520479289247763/config.xml
	at org.jvnet.hudson.test.TemporaryDirectoryAllocator.delete(TemporaryDirectoryAllocator.java:144)
	at org.jvnet.hudson.test.TemporaryDirectoryAllocator.delete(TemporaryDirectoryAllocator.java:131)
	at org.jvnet.hudson.test.TemporaryDirectoryAllocator.delete(TemporaryDirectoryAllocator.java:131)
	at org.jvnet.hudson.test.TemporaryDirectoryAllocator.dispose(TemporaryDirectoryAllocator.java:99)
	at org.jvnet.hudson.test.TestEnvironment.dispose(TestEnvironment.java:84)
	at org.jvnet.hudson.test.JenkinsRule.after(JenkinsRule.java:527)
	at org.jvnet.hudson.test.JenkinsRule$1.evaluate(JenkinsRule.java:665)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.lang.Thread.run(Thread.java:840)
Caused by: java.nio.file.DirectoryNotEmptyException: [REDACTED]/target/tmp/j h16565971146678333138/users/Fred_10744520479289247763
	at java.base/sun.nio.fs.UnixFileSystemProvider.implDelete(UnixFileSystemProvider.java:246)
	at java.base/sun.nio.fs.AbstractFileSystemProvider.deleteIfExists(AbstractFileSystemProvider.java:110)
	at java.base/java.nio.file.Files.deleteIfExists(Files.java:1191)
	at org.jvnet.hudson.test.TemporaryDirectoryAllocator.delete(TemporaryDirectoryAllocator.java:141)
	... 8 more
```

After:

```
java.nio.file.DirectoryNotEmptyException: [REDACTED]/target/tmp/j h16565971146678333138/users/Fred_10744520479289247763
        at java.base/sun.nio.fs.UnixFileSystemProvider.implDelete(UnixFileSystemProvider.java:289)
        at java.base/sun.nio.fs.AbstractFileSystemProvider.deleteIfExists(AbstractFileSystemProvider.java:109)
        at java.base/java.nio.file.Files.deleteIfExists(Files.java:1191)
        at org.jvnet.hudson.test.TemporaryDirectoryAllocator.delete(TemporaryDirectoryAllocator.java:142)
        at org.jvnet.hudson.test.TemporaryDirectoryAllocator.dispose(TemporaryDirectoryAllocator.java:100)
        at org.jvnet.hudson.test.TestEnvironment.dispose(TestEnvironment.java:84)
        at org.jvnet.hudson.test.JenkinsRule.after(JenkinsRule.java:586)
        at org.jvnet.hudson.test.JenkinsRule$1.evaluate(JenkinsRule.java:724)
        at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:317)
        at java.base/java.lang.Thread.run(Thread.java:1583)
        Suppressed: java.io.IOException: These files still exist : config.xml
                at org.jvnet.hudson.test.TemporaryDirectoryAllocator.delete(TemporaryDirectoryAllocator.java:146)
                ... 6 more
```

src/main/java/org/jvnet/hudson/test/TemporaryDirectoryAllocator.java

@@ -36,6 +36,7 @@
 import java.util.logging.Logger;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
+import org.apache.commons.lang.StringUtils;
 
 /**
  * Allocates temporary directories and cleans it up at the end.
@@ -140,9 +141,11 @@ private void delete(Path p) throws IOException {
             }
             Files.deleteIfExists(p);
         } catch (DirectoryNotEmptyException x) {
+            String pathString = p.toString();
             try (Stream<Path> children = Files.list(p)) {
-                throw new IOException(children.map(Path::toString).collect(Collectors.joining(" ")), x);
+                x.addSuppressed(new IOException("These files still exist : " + children.map(Path::toString).map(s -> StringUtils.removeStart(s, pathString + File.separator)).collect(Collectors.joining(", "))));
             }
+            throw x;
         }
     }