Consistent error messages that are started by lower-case character
Consistently use "Internal Compiler Error", rather than "ICE" that is not widely known
Consistency use "CTFE internal error" for the errors that happened during interpretation

Maybe, some of the errors would be caused by the AST that comes from the semantic analysis stage and the CTFE interpreter does not intent. But "CTFE error" is much understandable for the compiler end users.
Merge pull request #4186 from 9rnsr/fix_errmsg

Improve consistency of dmd error messages