fix: Handle errors in flagState conversion

[thomaspoignant]

Description

When not setting a targetingKey and calling allFlagState we had an error saying that the TYPE_MISMATCH which was not explicit enough to understand the issue.

Not we are returning TARGETING_KEY_MISSING which is more explicit.

Closes issue(s)

Resolve #2453

Checklist

• I have tested this code
• I have added unit test to cover this code
• I have updated the documentation (README.md and /website/docs)
• I have followed the contributing guide

[netlify]

✅ Deploy Preview for go-feature-flag-doc-preview canceled.

Name	Link
🔨 Latest commit	092d3e5
🔍 Latest deploy log	https://app.netlify.com/sites/go-feature-flag-doc-preview/deploys/6700188bcee72200085452bd

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.56%. Comparing base (b5144e3) to head (092d3e5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2455      +/-   ##
==========================================
- Coverage   86.02%   85.56%   -0.47%     
==========================================
  Files         102      103       +1     
  Lines        3801     3816      +15     
==========================================
- Hits         3270     3265       -5     
- Misses        405      425      +20     
  Partials      126      126              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud