Don't include "error_code.h" in edify/expr.h.

Use forward declartion to avoid pull in the module that contains
error_code.h (trying to move it into libotautil). Otherwise all the
modules that include "edify/expr.h" need to depend on the module that
exports error_code.h.

.cpp sources should include "error_code.h" explicitly to use the enums.

Test: lunch aosp_{angler,bullhead,dragon,fugu,sailfish}-userdebug;
      mmma bootable/recovery
Change-Id: Ic82db2746c7deb866e8cdfb3c57e0b1ecc71c4dc