Use the `noexcept` members of `boost::filesystem::recursive_directory_iterator` in order to ignore `boost::filesystem::directory_iterator::construct: Permission denied` errors. The errors are logged though.

Steps to reproduce the issue:

```sh
# 1. create directory for -walletdir without read access:
mkdir /tmp/foo && chmod a-r /tmp/foo

# 2. run bitcoin-qt and should print an error, but continues running:
/Volumes/Bitcoin-Core/Bitcoin-Qt.app/Contents/MacOS/Bitcoin-Qt -regtest -walletdir=/tmp/foo
/private/tmp/foo: Permission denied

# 4. go to File -> Open Wallet and should segfault:
EXCEPTION: N5boost10filesystem16filesystem_errorE
boost::filesystem::directory_iterator::construct: Permission denied: "/private/tmp/foo"
bitcoin in Runaway exception
```

ninja check check-functional