Raise error if the specified db, schema or warehouse doesn't exist

[smtakeda]

Description

Fixed #110

This change will raise errors if the specified db, schema or warehouse doesn't exist at connection time. The role is already validated, so others should be too.

Checklist

• Code compiles correctly
• Run make fmt to fix inconsistent formats
• Run make lint to get lint errors and fix all of them
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary

[smtakeda]

Also error code issue was fixed too. Somehow iota doesn't work to assign the continuous ids for errors but all were set to the same number...

[codecov-io]

Codecov Report

Merging #124 into master will decrease coverage by 0.07%.
The diff coverage is 89.65%.

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
- Coverage   82.02%   81.94%   -0.08%     
==========================================
  Files          18       18              
  Lines        2359     2382      +23     
==========================================
+ Hits         1935     1952      +17     
- Misses        336      340       +4     
- Partials       88       90       +2

Impacted Files	Coverage Δ
errors.go	78.57% <ø> (ø)	⬆️
auth.go	90.37% <100%> (ø)	⬆️
driver.go	77.77% <88.88%> (+2.48%)	⬆️
connection.go	75.33% <0%> (-1.35%)	⬇️

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