Improve error message when invalid connection

[byersiiasa]

Minor improvement to error message, including add brackets

Please confirm that this PR has done the following:

• [ ] Tests Added
• [ ] Documentation Added
• [ ] Name of contributors Added to AUTHORS.rst
• [ ] Description in RELEASE_NOTES.md Added

Adding to RELEASE_NOTES.md (remove section after adding to RELEASE_NOTES.md)

Please add a single line in the release notes similar to the following:

- (#XX)[http://link-to-pr.com] Added feature which does something

Description of PR

Improve error message ValueError: You do not have access to instance xxxxx or it does not exist. UseConnection.valid_connectionsfor a list of accessible services.

changed to pyam.iiasa.Connection().valid_connections

[codecov[bot]]

Codecov Report

Merging #725 (da16668) into main (bec0295) will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #725   +/-   ##
=====================================
  Coverage   95.0%   95.0%           
=====================================
  Files         59      59           
  Lines       6003    6003           
=====================================
  Hits        5707    5707           
  Misses       296     296           

Impacted Files	Coverage Δ
pyam/iiasa.py	86.7% <ø> (ø)

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