Return a proper error message when executing `schema load` with the wrong credential

[dgarros]

Fixes #127

I also have a PR coming to improve the error codes and the message returned by the backend

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
infrahub_sdk/schema.py	0.00%	0 Missing and 1 partial :warning:

@@            Coverage Diff             @@
##           stable     #144      +/-   ##
==========================================
- Coverage   65.40%   65.34%   -0.06%     
==========================================
  Files          76       76              
  Lines        6923     6940      +17     
  Branches     1367     1374       +7     
==========================================
+ Hits         4528     4535       +7     
- Misses       2028     2032       +4     
- Partials      367      373       +6     

Flag	Coverage Δ
python-3.10	44.32% <0.00%> (-0.08%)	:arrow_down:
python-3.11	44.32% <0.00%> (-0.08%)	:arrow_down:
python-3.12	44.32% <0.00%> (-0.08%)	:arrow_down:
python-3.9	43.44% <0.00%> (-0.08%)	:arrow_down:
python-filler-3.12	23.86% <0.00%> (+0.01%)	:arrow_up:

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Files with missing lines	Coverage Δ
infrahub_sdk/schema.py	73.89% <0.00%> (+0.27%)	:arrow_up:

... and 3 files with indirect coverage changes