chore: Change the error message when transfer fails because of no assignement to avoid confusion

[KevLehman]

Proposed changes (including videos or screenshots)

Issue(s)

https://rocketchat.atlassian.net/browse/CTZ-287

Steps to test or reproduce

Further comments

This is to avoid confusion. A transfer could fail even when there are online agents but they're over their limits for taking chats, so saying "no online agents" is misleading.

Now, the message is changed to "not available for service" to indicate they're just not available.

[dionisio-bot]

Looks like this PR is not ready to merge, because of the following issues:

• This PR is targeting the wrong base branch. It should target 7.10.0, but it targets 7.9.0

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[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.80%. Comparing base (4ad328f) to head (2705d99).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #36528   +/-   ##
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  Coverage    65.80%   65.80%           
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  Files         3188     3188           
  Lines       106922   106922           
  Branches     20389    20391    +2     
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+ Hits         70357    70364    +7     
+ Misses       33892    33886    -6     
+ Partials      2673     2672    -1     

Flag	Coverage Δ
e2e	58.11% <ø> (-0.01%)	⬇️
unit	70.94% <ø> (+0.01%)	⬆️

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