Report why fetching an IANA bootstrap file failed

[tomsommer]

Related to #4.

When file_get_contents() fails to download an IANA bootstrap file, AbstractRdapService::fromURL() swallowed the PHP warning and threw:

Protocol service URL "https://data.iana.org/rdap/dns.json" return invalid data

That made #4 hard to diagnose; the real cause there was allow_url_fopen=0. Now the first PHP warning is included:

Could not fetch protocol service URL "https://data.iana.org/rdap/dns.json": file_get_contents(): https:// wrapper is disabled in the server configuration by allow_url_fopen=0

The exception class is still InvalidServiceDefinitionException, so existing catch blocks keep working. Content that downloads but is not a valid bootstrap file still produces the "return invalid data" message.

With allow_url_fopen disabled, the bootstrap files and RDAP queries can now go through a PSR-18 client instead (#6):

$client = new Client(new Psr18Client());

phpcs and phpstan pass; checked with php -d allow_url_fopen=0 and with it enabled.