fix: update to unable to find conversion rate when failing to get conversion rate

[salimtb]

Description

Currently we are displaying "unable to load balance" whenever the app is not able to get conversion rates for an asset. To be more accurate we should update the message to say "unable to find conversion rate" for this scenario.

Related issues

Fixes: #8151

Manual testing steps

1. Go to the wallet view
2. import uniswap token in sepolia testnet
3. app is not able to retrieve conversion rate
4. "unable to load conversion rates" should be displayed (FAILED: "unable to load balance" is displayed)

Screenshots/Recordings

Before

After

Pre-merge author checklist

• I’ve followed MetaMask Coding Standards.
• I've clearly explained what problem this PR is solving and how it is solved.
• I've linked related issues
• I've included manual testing steps
• I've included screenshots/recordings if applicable
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.
• I’ve properly set the pull request status:
  • In case it's not yet "ready for review", I've set it to "draft".
  • In case it's "ready for review", I've changed it from "draft" to "non-draft".

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

[github-actions]

CLA Signature Action: All authors have signed the CLA. You may need to manually re-run the blocking PR check if it doesn't pass in a few minutes.

[github-actions]

E2E test started on Bitrise: https://app.bitrise.io/app/be69d4368ee7e86d/pipelines/3679f453-e69b-444c-b8d5-2fa79ed1918a
You can also kick off another Bitrise E2E smoke test by removing and re-applying the (Run Smoke E2E) label

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b1271ef) 39.56% compared to head (d82db03) 39.60%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8184      +/-   ##
==========================================
+ Coverage   39.56%   39.60%   +0.04%     
==========================================
  Files        1231     1232       +1     
  Lines       29821    29826       +5     
  Branches     2841     2841              
==========================================
+ Hits        11800    11814      +14     
+ Misses      17327    17322       -5     
+ Partials      694      690       -4     

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[sethkfman]

LGTM. I kicked off E2E one more time.

[github-actions]

E2E test started on Bitrise: https://app.bitrise.io/app/be69d4368ee7e86d/pipelines/61aef386-2d0e-4a63-9803-b5c89d0258bf
You can also kick off another Bitrise E2E smoke test by removing and re-applying the (Run Smoke E2E) label

[sonarcloud]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud