fix: Test connection before starting on create transaction

[hughhhh]

SUMMARY

Small bug where on failure of db connection, service fails to rollback. To fix this i've put the db connection before running on create database

TESTING INSTRUCTIONS

1. Goto Database Connection modal
2. Enter bad sqlalchemy_uri
3. See failure raise in toast
4. Refresh page and make sure the db wasn't add to the list

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #15100 (646a47f) into master (f8b270d) will decrease coverage by 0.14%.
The diff coverage is 85.24%.

❗ Current head 646a47f differs from pull request most recent head 05d310d. Consider uploading reports for the commit 05d310d to get more accurate results

@@            Coverage Diff             @@
##           master   #15100      +/-   ##
==========================================
- Coverage   77.53%   77.38%   -0.15%     
==========================================
  Files         967      967              
  Lines       49740    49752      +12     
  Branches     6351     6351              
==========================================
- Hits        38565    38500      -65     
- Misses      10973    11050      +77     
  Partials      202      202              

Flag	Coverage Δ
hive	?
mysql	81.71% <90.90%> (+<0.01%)	⬆️
postgres	81.73% <90.90%> (-0.01%)	⬇️
python	81.81% <90.90%> (-0.29%)	⬇️
sqlite	81.36% <68.18%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset-frontend/src/dashboard/actions/hydrate.js	1.72% <ø> (ø)
...nd/src/dashboard/containers/DashboardComponent.jsx	92.30% <ø> (ø)
...-frontend/src/dashboard/reducers/dashboardState.js	73.33% <0.00%> (ø)
...et-frontend/src/filters/components/Select/types.ts	100.00% <ø> (ø)
...t-frontend/src/dashboard/actions/dashboardState.js	28.65% <33.33%> (ø)
.../FilterBar/CascadeFilters/CascadePopover/index.tsx	61.76% <50.00%> (ø)
...board/components/nativeFilters/FilterBar/index.tsx	90.90% <83.33%> (ø)
superset/databases/commands/create.py	88.23% <83.33%> (-3.93%)	⬇️
...d/src/dashboard/components/gridComponents/Tabs.jsx	87.50% <88.88%> (ø)
superset/views/core.py	75.55% <88.88%> (+0.07%)	⬆️
... and 25 more

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[betodealmeida]

LGTM, but I wonder if we should add db.session.rollback() to the except block in line 70 now?