DRILL-6591: Show Exception for failed queries submitted in WebUI

When query fails on Web UI result page no error is shown, only "No result found." 
This was because DRILL-6477 (PR apache#1309) switched to `WebUserConnection.await(long timeoutInMillis)` . Unlike the original `WebUserConnection.await()`, this method did not throw any UserException generated by a query failure.
The fix was to add a new WebUser-only method - `WebUserConnection.timedWait(long timeoutInMillis)` 
This ensures that other callers to the `WebUserConnection.await(timeoutInMillis)` is unaffected.

exec/java-exec/src/main/java/org/apache/drill/exec/rpc/AbstractDisposableUserClientConnection.java

@@ -54,6 +54,19 @@ public boolean await(final long timeoutMillis) throws InterruptedException {
     return latch.await(timeoutMillis, TimeUnit.MILLISECONDS);
   }
 
+  /**
+   * Wait until the query has completed or timeout is passed. (Used only in case of WebUser)
+   *
+   * @throws Exception
+   */
+  public boolean timedWait(final long timeoutMillis) throws Exception {
+    boolean status = latch.await(timeoutMillis, TimeUnit.MILLISECONDS);
+    if (exception != null) {
+      throw exception;
+    }
+    return status;
+  }
+
   /**
    * Wait indefinitely until the query is completed. Used only in case of WebUser
    *

exec/java-exec/src/main/java/org/apache/drill/exec/server/rest/QueryWrapper.java

@@ -83,12 +83,17 @@ public QueryResult run(final WorkManager workManager, final WebUserConnection we
     float usagePercent = getHeapUsage();
 
     // Wait until the query execution is complete or there is error submitting the query
-    logger.debug("Wait until the query execution is complete or there is error submitting the query");
+    if (logger.isDebugEnabled()) {
+      logger.debug("Wait until the query execution is complete or there is error submitting the query");
+    }
     do {
       try {
-        isComplete = webUserConnection.await(TimeUnit.SECONDS.toMillis(1)); /*periodically timeout to check heap*/
-      } catch (Exception e) { }
-
+        isComplete = webUserConnection.timedWait(TimeUnit.SECONDS.toMillis(1)); /*periodically timeout 1sec to check heap */
+      } catch (Exception e) { /* Timed Out => Check usage */
+        if (e instanceof UserException) { //Throw UserException only
+          throw e;
+        }
+      }
       usagePercent = getHeapUsage();
       if (usagePercent >  HEAP_MEMORY_FAILURE_THRESHOLD) {
         nearlyOutOfHeapSpace = true;
@@ -97,17 +102,19 @@ public QueryResult run(final WorkManager workManager, final WebUserConnection we
 
     //Fail if nearly out of heap space
     if (nearlyOutOfHeapSpace) {
+      UserException almostOutOfHeapException = UserException.resourceError(
+          new Throwable(
+              "There is not enough heap memory to run this query using the web interface. "
+              + "Please try a query with fewer columns or with a filter or limit condition to limit the data returned. "
+              + "You can also try an ODBC/JDBC client. "
+              )
+          )
+        .build(logger);
+      //Add event
       workManager.getBee().getForemanForQueryId(queryId)
-        .addToEventQueue(QueryState.FAILED,
-            UserException.resourceError(
-                new Throwable(
-                    "There is not enough heap memory to run this query using the web interface. "
-                    + "Please try a query with fewer columns or with a filter or limit condition to limit the data returned. "
-                    + "You can also try an ODBC/JDBC client. "
-                    )
-                )
-              .build(logger)
-            );
+        .addToEventQueue(QueryState.FAILED, almostOutOfHeapException);
+      //Return NearlyOutOfHeap exception
+      throw almostOutOfHeapException;
     }
 
     if (logger.isTraceEnabled()) {