JIRA-91: fix UI

## UI crash

* invalid request path ( default 404 by WebAppContext )

* host/browse or host/browse? or host/browse?(anything except 'path') will crash due to NullPointerException 

* display file may crash due to IOException

* set invalid offset in display file will crash, offset should be numbers and should be in range [0, size(file)]

* host/VALIDPATH?xxxxx&xxx, if Servlet corresponding to VALIDPATH doesn't handle request parameter xxxx, just ignore ?xxxx&xxxx

I don't find logging these invalid URLs valuable, do I need to log any URL that's invalid even if I've handled them?