Report errors when opening config files

The error handling when opening config files was ignoring all errors
except ENOEXIST.  Report other errors, instead of passing nil to
json.NewDecoder and getting:
config file: out/soong/soong.config did not parse correctly: invalid argument

Bug: 73951413
Test: touch out/soong/soong.config && chmod a-r out/soong/soong.config && m
Test: rm out/soong/soong.config && m
Change-Id: I4a609b7f060b760b76ee829b83c0eb405340f58f