fromExternalizableId should just return null, not throw IAE, if the job has...

fromExternalizableId should just return null, not throw IAE, if the job has since been renamed/move/deleted.
(It could already return null if the run has since been deleted; #1264 failed to mark this properly.)
Reserving IAE for inputs that getExternalizableId could not actually have produced,
and making sure to also throw this in case of non-numeric text after the hash.
Adding Javadoc for both methods.

core/src/main/java/hudson/model/Run.java

@@ -2281,21 +2281,40 @@ public abstract class Run <JobT extends Job<JobT,RunT>,RunT extends Run<JobT,Run
         return env;
     }
 
+    /**
+     * Produces an identifier for this run unique in the system.
+     * @return the {@link Job#getFullName}, then {@code #}, then {@link #getNumber}
+     * @see #fromExternalizableId
+     */
     public @Nonnull String getExternalizableId() {
         return project.getFullName() + "#" + getNumber();
     }
 
-    public @Nonnull static Run<?,?> fromExternalizableId(String id) {
+    /**
+     * Tries to find a run from an persisted identifier.
+     * @param id as produced by {@link #getExternalizableId}
+     * @return the same run, or null if the job or run was not found
+     * @throws IllegalArgumentException if the ID is malformed
+     */
+    public @CheckForNull static Run<?,?> fromExternalizableId(String id) throws IllegalArgumentException {
         int hash = id.lastIndexOf('#');
         if (hash <= 0) {
             throw new IllegalArgumentException("Invalid id");
         }
         String jobName = id.substring(0, hash);
-        int number = Integer.parseInt(id.substring(hash + 1));
-
-        Job<?,?> job = Jenkins.getInstance().getItemByFullName(jobName, Job.class);
+        int number;
+        try {
+            number = Integer.parseInt(id.substring(hash + 1));
+        } catch (NumberFormatException x) {
+            throw new IllegalArgumentException(x);
+        }
+        Jenkins j = Jenkins.getInstance();
+        if (j == null) {
+            return null;
+        }
+        Job<?,?> job = j.getItemByFullName(jobName, Job.class);
         if (job == null) {
-            throw new IllegalArgumentException("no such job " + jobName);
+            return null;
         }
         return job.getBuildByNumber(number);
     }