Improve error message if something goes wrong when loading storage

[oyvindeide]

Bacport of: #8604

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure tests pass locally (after every commit!)

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.23810% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.77%. Comparing base (f0fb2aa) to head (1e39fdb).

Files with missing lines	Patch %	Lines
src/ert/gui/main.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           version-10.3    #8606      +/-   ##
================================================
- Coverage         90.79%   90.77%   -0.03%     
================================================
  Files               342      342              
  Lines             20945    20961      +16     
================================================
+ Hits              19017    19027      +10     
- Misses             1928     1934       +6     

Flag	Coverage Δ
gui-tests	75.61% <59.52%> (-0.21%)	⬇️
integration-tests	53.79% <42.85%> (-0.20%)	⬇️
unit-tests	68.77% <78.57%> (-0.02%)	⬇️

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[DanSava]

🚀