feat: format errors

[timcadman]

Implement two main changes:

1. Added argument return_error to datashield.assign functions, giving the option either to immediately return errors or to store them to be accessible with a call to datashield.errors
2. Added styling to the output returned by datashield.errors.

Note: tests written for datashield.assign.expr; however I was unable to create errors with either datashield.assign.table or datashield.assign.resource. If you have code which will do this tell me and I'll add tests.

[ymarcon]

Not tested yet, please fix dependencies and documentation. Also I expect the man folder to be updated. In case you do not know how to do it in Rstudio, make sure in the project options that all the boxes are checked:

so that documentation is generated at check time.

[timcadman]

Not tested yet, please fix dependencies and documentation. Also I expect the man folder to be updated. In case you do not know how to do it in Rstudio, make sure in the project options that all the boxes are checked:

so that documentation is generated at check time.

Sorry Yannick, not sure what happened yesterday ... trying to do 17 things at once!

[timcadman]

Hi Yannick, following your comments:

• I've now tested this with an external script (tests added to package).
• You were right to suggest this, as it highlighted an issue as currently I was returning warnings, not errors. I've tweaked further now so an error is returned and this gives the correct behaviour (throwing error when running a script).

[ymarcon]

Please rebase to master, there are conflicts in the NAMESPACE.

[ymarcon]

You are introducing a circular dependency by importing DSLite. You could move the tests to the DSLite package instead?