[roslaunch] Better exception handling when resource is not found.

When `$(find pkg)` fail to return a resource in `arg` tag, `roslaunch` fails without printing useful output. With this PR it provides better explanation.

Without this PR:
```
$ roslaunch /tmp/invalid_arg.launch
:
Invalid <arg> tag: foo
ROS path [0]=/opt/ros/kinetic/share/ros
ROS path [1]=/home/n130s/ROS/indigo_trusty/cws_rosdt/src/ros/ros_comm/tools/roslaunch
ROS path [2]=/opt/ros/kinetic/share

Arg xml is <arg default="$(find foo)/.config" name="foo"/>
The traceback for the exception was written to the log file
```

With this PR:
```
$ roslaunch /tmp/invalid_arg.launch
:
Invalid <arg> tag: Make sure the following is found in ROS_PACKAGE_PATH: foo
ROS path [0]=/opt/ros/kinetic/share/ros
ROS path [1]=/home/n130s/ROS/indigo_trusty/cws_rosdt/src/ros/ros_comm/tools/roslaunch
ROS path [2]=/opt/ros/kinetic/share

Arg xml is <arg default="$(find foo)/.config" name="foo"/>
The traceback for the exception was written to the log file
```

```
$ more /tmp/invalid_arg.launch
<?xml version="1.0"?>
<launch>
  <arg name="foo" default="$(find foo)/.config" />
  <arg name="baa" default="$(arg foo)/hoge.yaml" />
</launch>
```

tools/roslaunch/src/roslaunch/xmlloader.py

@@ -47,6 +47,7 @@
 from xml.dom import Node as DomNode #avoid aliasing
 
 from rosgraph.names import make_global_ns, ns_join, is_private, is_legal_name, get_ros_namespace
+from rospkg import ResourceNotFound
 
 from .core import Param, Node, Test, Machine, RLException
 from . import loader
@@ -301,6 +302,9 @@ def _arg_tag(self, tag, context, ros_config, verbose=True):
         except substitution_args.ArgException as e:
             raise XmlParseException(
                 "arg '%s' is not defined. \n\nArg xml is %s"%(e, tag.toxml()))
+        except ResourceNotFound as e:
+            raise XmlParseException(
+                "Invalid <arg> tag: Make sure the following is found in ROS_PACKAGE_PATH: %s \n\nArg xml is %s"%(e, tag.toxml()))
         except Exception as e:
             raise XmlParseException(
                 "Invalid <arg> tag: %s. \n\nArg xml is %s"%(e, tag.toxml()))