Update Error Message to Explain Namespace Restrictions for := and let in data.table

[Nj221102]

closes #5357.

Summary of Changes:
The error message related to the use of := and let within the data.table package has been updated to provide clearer guidance on namespace appending. The new error message now includes specific information that namespace appending such as data.table:::= is not allowed.

• The updated error message now reads:

  "Check that is.data.table(DT) == TRUE. Otherwise, :=, `:=`(...) and let(...) are defined for use in j, once only and in particular ways. Namespace appending such as data.table::`:=(...)` is not allowed. See help(\":=\")."
  

Details:
The functions := and let in data.table are designed to work with non-standard evaluation (NSE). When these functions are used with a namespace prefix (e.g., data.table:::=), the NSE mechanisms fail to operate as intended, leading to confusing error messages. The new error message is intended to directly address this misuse and guide users towards the correct usage patterns.

[github-actions]

Generated via commit 2e57461

Download link for the artifact containing the test results: ↓ atime-results.zip

Time taken to finish the standard R installation steps: 11 minutes and 20 seconds

Time taken to run atime::atime_pkg on the tests: 3 minutes and 25 seconds

[MichaelChirico]

Thanks!