FAL-1496: Catch xblock parsing exceptions on reading blocks

Description

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    1. Description

This MR is about rising a special `xblock.exceptions.XBlockParseException` on failed blocks parsing. It will allow avoiding impact on parent Block if child block caused issues while parsed.

Example: [labxchange-xblocks/pull/17/files](https://github.com/open-craft/labxchange-xblocks/pull/17/files#diff-cb15436eafd5444eb607d5c13803643614714c84fb796ad9eabe8510e87e4cf9R56-R59)

*Note:* unit tests fails because the xblock library wasn't yet bumped.

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    1. Supporting information

FAL-1496 OLX parsing should not impact parent XBlocks

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    1. Testing instructions

1. Install xblock from master into LMS/studio:
```
$ pip uninstall xblock
$ pip install git+https://github.com/open-craft/XBlock@s0b0lev/FAL-1496-olx-parsing#egg=xblock
```
2. Restart both studio/LMS
3. Update labxchange-xblock version
4. Run tests, changes shouldn't affect anything

Raise exception at AssignmentBlock.student_view_blcok

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    1. Deadline

None?

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    1. Other information

This change depends on this PR https://github.com/edx/XBlock/pull/516 (I will update the library version for edx-platform as soon as it will be bumped)

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Assignee

Unassigned

Reporter

Open Source Pull Request Bot

Labels

Contributor Name

Aleksandr Sobolev

Repo

edx/edx-platform

Customer

Github Lines Added

13

Github Lines Deleted

3