[bugfix] throw "TypeError: 'NoneType'..." when save chart to dash

[hitdemo]

All changes in save_or_overwrite_slice:

• fix bug
• change text from "slice" to "chart"
• make info localizable

---

Bug replication steps:

1. remove Admin's "can add on DashboardModelView" permission
2. open a chart in explore page, then click "save" -> "Add to new dashboard"
3. a exception is thrown

../incubator-superset/superset/views/core.py", line 1409, in save_or_overwrite_slice
    _('You don\'t have the rights to ') + _('create a ') + _('dashboard'),
TypeError: 'NoneType' object is not callable

it seems that in function save_or_overwrite_slice, lazy_gettext's alies _ is overridden by a local variable _

def save_or_overwrite_slice
        .....
        form_data, _ = self.get_form_data()
        .....

        .....
        _('You don\'t have the rights to ') + _('create a ') + _('dashboard'),
        .....

[codecov-io]

Codecov Report

Merging #5835 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #5835   +/-   ##
=======================================
  Coverage   64.46%   64.46%           
=======================================
  Files         422      422           
  Lines       20566    20566           
  Branches     2250     2250           
=======================================
  Hits        13257    13257           
  Misses       7182     7182           
  Partials      127      127

Impacted Files	Coverage Δ
superset/views/core.py	74.91% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b210742...b5152b2. Read the comment docs.

[mistercrunch]

oh interesting, I never thought that the gettext standard of using _ conflicts with the python standard for unused assignments

[mistercrunch]

LGTM