Issue 1784: Catch template parsing errors and show useful message.

[Issue 1784](http://code.google.com/p/ankidroid/issues/detail?id=1784)

Show a useful error message with some instructions when the template cannot be parsed. It ends up looking like this:
![screenshot-1371997727225](https://f.cloud.github.com/assets/789082/692261/26040174-dc13-11e2-8637-140c808ad2ed.png)

Since it isn't always obvious what the error might be, even on the desktop client, I thought it would be useful to also include the error in the output.