pkg/cover: don't fail when can't open single source file
pkg/cover: don't fail when can't open single source file

Ignore individual errors of opening/locating source files
because there is a number of reasons when/why it can happen.
We fail only if we can't open/locate any single source file.
syz-ci can mess state of source files (https://github.com/google/syzkaller/issues/1770),
or bazel lies about location of auto-generated files,
or a used can update source files with git pull/checkout.

Update #1770

xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller