Name unreadable spec files and keep validating the rest - #525

Problem

When validating multiple spec files, a file that cannot be read (missing
path, directory, unreadable file) aborts the whole run in two unhelpful
ways:

1. The error message names no file: main() prints the bare exception
   (No such file: foo.yaml), so with several inputs you cannot tell
   which one failed.
2. sys.exit(1) fires immediately, so the files listed after the bad one
   are never validated.

Repro

openapi-spec-validator i_dont_exist.yaml petstore.yaml

Before: prints No such file: i_dont_exist.yaml and exits; petstore.yaml
is never checked.

Fix

In openapi_spec_validator/__main__.py, report read failures through the
existing print_error(filename, exc) helper (so the message carries the
file name), keep validating the remaining files, and exit nonzero at the
end if any input failed to read. Validation-error paths are untouched, and
the single-file case still exits nonzero with the same message text.

Tests

Added test_read_error_names_file_and_continues to
tests/integration/test_main.py: a missing file followed by a valid spec
prints i_dont_exist.yaml: Error: ... AND petstore.yaml: OK, then exits
nonzero. Fails before the fix (valid file never processed), passes after.
Full tests/integration/test_main.py: 23 passed.