Validate input to alert the user when we can't parse it as BODS

[stevenday]

Currently, the tool just errors if what you paste isn't a JSON array that looks like BODS statements. Long term, we could look to use https://datareview.openownership.org to properly validate the data, but it would be a useful UX improvement just to catch parsing errors and tell the user something went wrong, rather than silently erroring.