[EuiCodeEditor] Custom Modes

[jloleysens]

It looks like, when using a custom mode with the EuiCodeEditor component a lookup is still happening for a mode which results in this error in the console:

See this Kibana PR fixing another EuiCodeEditor issue I initially reported here: #2517 but since realised part of the issue I should address in Kibana. For an example of this issue in Kibana, see this code:

https://github.com/elastic/kibana/blob/167dd7f5a001a9d1ecc48f0125d5b81999f620a6/x-pack/legacy/plugins/grokdebugger/public/sections/grokdebugger/components/pattern_input/pattern_input.js#L34