Fix accessibility and consistency 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 have several problems:

• 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.

• On Firefox, the inaccessible text colour affects the placeholder colour when the input is empty. Using additional styles will also affect the empty input element. However, the input element will retain the presence or absence of the error state that its previous contents caused. It should not have the error styles whenever it is empty.