Report more kinds of coerce_fail reasons.

compiler/type_assign.m:
    Split one coerce_fail into three parts, to simplify the generation
    of diagnostics that are specialized to their situations.

compiler/typecheck_errors.m:
    Generate diagnostics for several coerce_fails, including the new ones.

compiler/typecheck_coerce.m:
    Record the new kinds of coerce_fails.

compiler/typecheck_coerce.m:
compiler/typecheck_util.m:
    Move a predicate and its associated types from typecheck_coerce.m
    to typecheck_util.m to allow the new code in typecheck_errors.m
    to access it.

tests/invalid/coerce_type_error.err_exp:
tests/invalid/coerce_unify_tvars.err_exp:
    Expect the new diagnostics.