broaden a 'catch' to catch other failures showing all headers.

Market reported NPE:

java.lang.NullPointerException
at com.fsck.k9.view.MessageHeader.showAdditionalHeaders(MessageHeader.java:186)
at com.fsck.k9.view.MessageHeader.onShowAdditionalHeaders(MessageHeader.java:318)
at com.fsck.k9.fragment.MessageViewFragment.onToggleAllHeadersView(MessageViewFragment.java:300)
at com.fsck.k9.activity.MessageList.onOptionsItemSelected(MessageList.java:867)
at android.support.v4.app.Watson.onMenuItemSelected(Watson.java:119)
at com.actionbarsherlock.ActionBarSherlock.callbackOptionsItemSelected(ActionBarSherlock.java:603)
at com.actionbarsherlock.internal.ActionBarSherlockNative.dispatchOptionsItemSelected(ActionBarSherlockNative.java:78)
at com.actionbarsherlock.app.SherlockFragmentActivity.onMenuItemSelected(SherlockFragmentActivity.java:205)
at com.android.internal.policy.impl.PhoneWindow.onMenuItemSelected(PhoneWindow.java:1047)
at com.android.internal.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:735)
at com.android.internal.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:149)
at com.android.internal.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:874)
at com.android.internal.view.menu.ListMenuPresenter.onItemClick(ListMenuPresenter.java:180)
at android.widget.AdapterView.performItemClick(AdapterView.java:301)
at android.widget.AbsListView.performItemClick(AbsListView.java:1276)
at android.widget.AbsListView$PerformClick.run(AbsListView.java:3067)
at android.widget.AbsListView$1.run(AbsListView.java:3963)
at android.os.Handler.handleCallback(Handler.java:615)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4898)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1006)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:773)
at dalvik.system.NativeStart.main(Native Method)