plots/metrics/params/experiments show: stop throwing exceptions (#5984)

* params/metrics/plots/exp show: introduce onerror

* fixup

* plots/metrics/params: removing errors

* removing more errors

* removing errors

* some more removing

* new plots ux

* new ux for metrics

* new ux for params

* fixing tests

* fixups

* minor test fixes

* minor test fixes

* commands: plots/params: fix ui imports

* working on error handlng decorator, stopped at show checkpoints

* error_handler, further dvelopment

* plots just before moving loading to collect

* appending errors to results: initial passing

* fixup

* onerror: remove onerror fixture

* fixups

* plots/metrics/params/experiments: temporarily remove error warnings

* plots: extract parsing out of class method

* metrics/experiments: provide exception serializer for show-json

* params/plots/metrics: signal errors

* rebase fixups

* fix TODOs

* fixup

* skshetry's review refactor

tests/unit/command/test_params.py

@@ -48,7 +48,12 @@ def test_params_diff_from_cli(dvc, mocker):
     assert cmd.run() == 0
 
     params_diff.assert_called_once_with(
-        cmd.repo, a_rev=None, b_rev=None, all=False, targets=None, deps=False
+        cmd.repo,
+        a_rev=None,
+        b_rev=None,
+        all=False,
+        targets=None,
+        deps=False,
     )
     show_diff_mock.assert_called_once_with(
         {}, title="Param", markdown=False, no_path=False, show_changes=False