improve error indication in obfuscation dialog, streamline pinned gateways a little bit

.../bitmaskclient/base/fragments/ObfuscationProxyDialog.java

...ain/java/se/leap/bitmaskclient/base/models/Transport.java

...va/se/leap/bitmaskclient/base/utils/PreferenceHelper.java

app/src/main/java/se/leap/bitmaskclient/eip/Gateway.java

.../main/java/se/leap/bitmaskclient/eip/GatewaysManager.java

...in/java/se/leap/bitmaskclient/eip/VpnConfigGenerator.java