Add --no-file-errors flag to suppress logging of errors opening/searching files

Many use cases will not care about files which can't be read and would
like to ignore those errors (such as `ag foo /tmp` on a shared host).
Shell redirection of stderr is an option, but it indiscriminately throws
out all errors.

The behaviour of --no-file-errors is similar to grep's (ambiguously
named) --no-messages flag.  While copying the short and long option
names from grep benefits users familiar with the grep option, the short
option (-s) is already used by ag.  With the short option unavailable,
renaming the long option to be more clear makes more sense.