[Chapel Merge] Change internal error to user error for bad import

Branch: refs/heads/main
Revision: cb43332
Author: bradcray
Link: Unavailable
Log Message:

Merge pull request #18731 from bradcray/user-error-bad-import-2

Change internal error to user error for bad import expression

[reviewed by @dlongnecke-cray]

This resolves an error pointed out by @cassella in which an import
statement with an unexpected expression caused an internal error
rather than a helpful user-facing one. Changed to a user error and
added Paul's test to prevent backsliding.

Resolves #18718.

Modified Files:
A test/statements/errors/importIllegalExpr.chpl

A test/statements/errors/importIllegalExpr.good
M compiler/AST/ImportStmt.cpp

Compare: https://github.com/chapel-lang/chapel/compare/9c28cd26a106...cb4333275a17