pass on IOException from Runtime.exec as the old code did - and as promised by the javadocs. This fixes ant-contrib's tests in Gump.

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@782990 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/taskdefs/Execute.java

@@ -825,6 +825,8 @@ public class Execute {
                         + Commandline.describeCommand(cmd), Project.MSG_DEBUG);
                 }
                 return Runtime.getRuntime().exec(cmd, env, workingDir);
+            } catch (IOException ioex) {
+                throw ioex;
             } catch (Exception exc) {
                 // IllegalAccess, IllegalArgument, ClassCast
                 throw new BuildException("Unable to execute command", exc);