Misleading error messages for assignments and dereferences

[Felalolf]

(created for PR)

In the following code, the first and second assignment fail as exhale and call error respectively:

func assignment() int {
    var x@, y [10]int
    exhale acc(&x)
    x = y
}

func load() int {
    var x@, y [10]int
    exhale acc(&x)
    y = x
}