jinja2: removed excessive context from Jinja2 raise/assert statements

[oliver-sanders]

User request: Make the Jinja2 raise / assert exceptions cleaner.

The raise and assert helpers are used by some workflow developers to allow validation of inputs / environment, e.g:

#!jinja2

{{ assert(SITE is defined, 'Please define the SITE variable') }}

The workflow developer configures the condition and a user-friendly message to display.

Unfortunately, that message gets lost in the resulting message:

$ cylc val .
Jinja2Error: Jinja2 Assertion Error: message
File /var/tmp/tmp.F4DJDykBo2/flow.cylc
  #!Jinja2
  
  {{ assert(SITE is defined, 'message') }}	<-- Exception

This scary looking error makes it look like something is genuinely wrong:

• The traceback looks like program code, it's not something the user is expected to understand.
• The Jinja2Error exception makes it look like an error.

This PR changes the exception from Jinja2Error to InputError and remove the extraneous context:

$ cylc val .
InputError: message
(add --verbose for more context)

The full exception is still available to workflows developers in verbose mode (can be helpful in tracking down the location of the assert / raise statement that caused the error).

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

Looks good, couple of related but simple test failures.

[hjoliver]

tests/f/jinja2/10-builtin-functions.t failing

[oliver-sanders]

(Dammit, thought I'd removed that)

[wxtim]

[Not a blocker for this PR]
This breaks Cylc Rose tests. I'll fix it and put a PR up after lunch

FAILED tests/functional/test_pre_configure.py::test_validate_fail[template variable not set] - assert []

[wxtim]

This is a really, really good idea IMO.

[oliver-sanders]

This breaks Cylc Rose tests. I'll fix it and put a PR up after lunch

Of course, I completely forgot those tests were using assert! Good catch!

[oliver-sanders]

Merging with two approvals.