Add custom error page for uncaught exceptions

[kitchoi]

Uncaught error such as #143 causes a plain "HTTP 500: Internal Server Error" page. This PR overloads tornado's write_error so that error message is displayed.

For #143, it displays something like this:

[codecov-io]

Current coverage is 88.61% (diff: 94.11%)

Merging #159 into master will increase coverage by 0.06%

@@             master       #159   diff @@
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  Files            36         36          
  Lines          1275       1291    +16   
  Methods           0          0          
  Messages          0          0          
  Branches        121        122     +1   
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+ Hits           1129       1144    +15   
  Misses          108        108          
- Partials         38         39     +1   

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[stefanoborini]

I'm not sure it's a good idea to leak internal information that way. I would leave the details for the internal log, but stay generic for the presented page.

[kitchoi]

This error page captures all uncaught exception, any generic exception. If the content of the exception is not displayed, then it is no different from the original behaviour of a plain 500 server error page.

[stefanoborini]

and that's the point. You don't want to leak internal information to externals. That way anyone can see exactly what went wrong and the path information. It is information leakage http://projects.webappsec.org/w/page/13246936/Information%20Leakage

[kitchoi]

@stefanoborini better?

[kitchoi]

@stefanoborini I know this is not the best place for an HTTPError, but I would prefer improving on this after the PR list is shorter.

[kitchoi]

And this is how it looks to the admin

[I 160726 12:06:00 orm:131] Creating session to db: sqlite:///remoteappmanager.db
[E 160726 12:06:00 application:118] The database is not initialised properly.
    Traceback (most recent call last):
      File "/home/kit/.virtualenv/simremote/lib/python3.4/site-packages/traitlets/traitlets.py", line 501, in get
        value = obj._trait_values[self.name]
    KeyError: 'user'

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "/home/kit/.virtualenv/simremote/lib/python3.4/site-packages/traitlets/traitlets.py", line 501, in get
        value = obj._trait_values[self.name]
    KeyError: 'db'

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "/mnt/hgfs/VM_shared/SimPhoNy/s2imphony-remote/remoteappmanager/application.py", line 115, in _db_default
        return cls(**self.file_config.accounting_kwargs)
      File "/mnt/hgfs/VM_shared/SimPhoNy/s2imphony-remote/remoteappmanager/db/orm.py", line 176, in __init__
        self.check_database_readable()
      File "/mnt/hgfs/VM_shared/SimPhoNy/s2imphony-remote/remoteappmanager/db/orm.py", line 190, in check_database_readable
        'Sqlite database {} is not readable'.format(file_path))
    OSError: Sqlite database /mnt/hgfs/VM_shared/SimPhoNy/s2imphony-remote/jupyterhub/remoteappmanager.db is not readable
[E 160726 12:06:00 logging_mixin:26] Internal Server Error : HTTP 500: The database is not initialised properly.. Ref: efc99ca8-5320-11e6-b741-000c29ae0276

[stefanoborini]

This code shows up again and again in all our tests. It's becoming a liability.

[itziakos]

It is always tricky to have to cleanup/recover the enviroment variables. Can you open an issue, this will need to be investigated.

[stefanoborini]

It's already there.