Ensure that any changes made by user in an open workspace are not
overridden when calling `bst workspace open` again with `--force` and
`--no-checkout` options.

When `--no-checkout` flag is used, BuildStream assumes that the target
destination already contain the sources (potentially edited already).
Hence BuildStream must not remove its contents.

Fixes #1086.

---

Also, fix a minor typo in the file that probably doesn't need a commit
of its own.

_exceptions.py: Align LoadError() parameter ordering

See merge request BuildStream/buildstream!1489

All of the errors which subclass from BstError have their first
positional argument as message, LoadError should follow this
ordering for consistency.

Mock BST_TEST_SUITE env when running update.py

Closes #1074

See merge request BuildStream/buildstream!1468

Return if there is no result from parsing the element

If this is not present in the env the cli will throw an exception
closes #1074

Fixing typos: comand corrected to command

See merge request BuildStream/buildstream!1486

Store Platform reference in Context instance variable

See merge request BuildStream/buildstream!1480