Invalid validation error message when using multi-line string

### HCL Template
```hcl

---snip---
variable "UserSetting" {
type = string
validation {
condition = can(regex("^((?:.*[a-zA-Z0-9])|(?:.*[!@#$%^&*])|(?:.*[0-9]))(?:.{8,})", var.UserSetting))
error_message = <

Start by reproducing the validation failure from the HCL example in deployments/4.0.0/cluster_vars.tf with a plan or apply. Trace the validation error-message handling to determine why a multi-line string is rejected, then verify that multi-line messages are accepted and the English-sentence punctuation requirement is removed or revised as requested.

Written by the indexing model from the issue text.