Better error handling for explorations with missing interactions

What steps will reproduce the problem?
1. Create new exploration and immediately go to preview mode without adding an 
interaction to the first state

OR

2. preview an exploration after deleting a state's interaction

Both of these situations lead to a warning that a state does not have an 
interaction. Since private explorations have non-strict validation and 
non-strict validation has been updated to allow null interactions, it's 
possible for an exploration to be viewed both in editor preview mode and 
learner view such that a state can be entered that does not have an 
interaction. This leads to the warning message "Error: No interaction 
specified." to the user.

In order to fix this, the following are suggested:

1. The above error message should be improve to explain that a specified state 
name does not have an interaction specified for it.
2. Learner view should only show explorations that pass strict-validation 
(regardless of if they're published). If they do not pass strict-validation, 
show a single error for all validation failures that say the exploration is 
broken and to contact the owner. Concatenated onto this shared error message 
would be the actual error reported by the validation function.
3. Show the feedback form for an exploration if it cannot be loaded correctly 
so users may easily leave feedback that it's broken.
4. Add error message at the top of preview mode that tells the user the 
exploration will not work at all in learner view if they do not fix the 
warnings/errors listed.
5. Stretch: automatically send feedback to a creator if the exploration fails 
validation in learner view.

These changes will also help the movement toward automatic draft saving.

Original issue reported on code.google.com by bhenn...@google.com on 2 Jun 2015 at 1:07

Dev4X / oppia

Better error handling for explorations with missing interactions #798