Better wrong input handling for Keycloak

[nordicdyno]

prints:

• 'Invalid username or password.' instead of 'unable to locate saml response field' in case of password valiadation failed
• 'otp code is invalid' instead of 'unable to locate saml response field' in case of otp code is invalid

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 45.45455% with 24 lines in your changes missing coverage. Please review.

Project coverage is 28.16%. Comparing base (99d6fe4) to head (622315b). Report is 150 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/provider/keycloak/keycloak.go	0.00%	15 Missing :warning:
pkg/provider/keycloak/otp_validator.go	68.96%	6 Missing and 3 partials :warning:

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1404       +/-   ##
===========================================
- Coverage   42.19%   28.16%   -14.04%     
===========================================
  Files          54       71       +17     
  Lines        6456    12585     +6129     
===========================================
+ Hits         2724     3544      +820     
- Misses       3283     8561     +5278     
- Partials      449      480       +31     

Flag	Coverage Δ
unittests	28.16% <45.45%> (-14.04%)	:arrow_down:

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[nordicdyno]

what do you think about merging this to master? are any improvements needed, or there are reasons not to merge?