Zikula_EntityAccess: Also find getter methods named `isField()` and not only `getField()`.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ---
| Refs tickets  | ---
| License       | MIT
| Doc PR        | ---
It now throws \RuntimeException's instead of the PHP Fatal Error it did before.