[DPE-5594] Ignore alias error in case alias is already existing

[dragomirp]

If the Posgresql and Pgbouncer charms are collocated on the same host, both will try to create a psql alias. The charm should ignore errors during alias creation, since the alias is not a critical component and collocation of both charms is a valid use case

Related to #636

[codecov]

Codecov Report

Attention: Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.

Project coverage is 70.81%. Comparing base (a8024fd) to head (f01b799).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/charm.py	55.55%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #637      +/-   ##
==========================================
- Coverage   70.89%   70.81%   -0.09%     
==========================================
  Files          12       12              
  Lines        3048     3053       +5     
  Branches      539      539              
==========================================
+ Hits         2161     2162       +1     
- Misses        771      775       +4     
  Partials      116      116              

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