Fix accessibility of Find and Replace/Go To Line error styles

[WebCoder49]

Error styles occur when a string not in the code is typed in the dialog for Find and Replace, or an invalid (too high, or too low, or wrong format) line number is typed into the dialog for Go To Line. They are in the class code-input_find-and-replace_error on the input element. They currently turn the text red, but:

The red on white has an inaccessible contrast. This needs to be improved (the lightest WCAG AAA contrast red against #ffffff is #b60000). However, error styles must still be distinguishable (though the text under the input that describes the error might help). Potentially set a background colour or red outline for the input as well as the text colour.

[WebCoder49]

On balance, it's important not to break user stylesheets for the dialogs, and the message under the input is accessible enough for colourblind people. Thus just a different contrasting text colour is now used as an error style.