fix: Make error message more clear about where ordering must be enabled when publishing

[kamalaboulhosn]

No description provided.

[codecov]

Codecov Report

Merging #293 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #293      +/-   ##
============================================
+ Coverage     79.18%   79.22%   +0.03%     
  Complexity      317      317              
============================================
  Files            21       21              
  Lines          2888     2888              
  Branches        155      155              
============================================
+ Hits           2287     2288       +1     
  Misses          537      537              
+ Partials         64       63       -1     

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/pubsub/v1/Publisher.java	87.96% <ø> (ø)	46.00 <0.00> (ø)
.../com/google/cloud/pubsub/v1/MessageDispatcher.java	85.36% <0.00%> (+0.48%)	26.00% <0.00%> (ø%)

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