grep: one file it cannot read is not the end of the search

`look.grep` aborted the whole walk and returned `OpenFailed` the moment any
file refused to open. One root-owned 0600 file in the tree — or one deleted
between the walk and the read, which is routine in a build tree — turned a
search of ten thousand files into zero results and a word on the message row
that explains nothing. A grep is a question about the files you can read, and
the ones you cannot are not an answer to it: they are skipped now, along with
a read that fails partway.

Opened NONBLOCK for the reason readFile has it, so a FIFO in the tree cannot
stop the search until somebody writes to it.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_016Q4RATpafkwahrovHQLKRf

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