Prevents SvgUri crashes when uri is invalid and adds fallback prop (#2071)

SvgUri component crashes an app when uri with SVG returns html or some other content instead of valid SVG.
This change prevents those crashes and adds fallback property of JSX type which is rendered instead of corrupted SVG.

USAGE.md

@@ -122,11 +122,13 @@ export default function TestComponent() {
 
 Both `SvgUri` and `SvgCssUri` log errors to the console, but otherwise ignore them.
 You can set property `onError` if you want to handle errors such as resource not
-existing in a different way.
+existing in a different way and `fallback` if you want to render another component 
+instead in such case.
 
 ```jsx
 import * as React from 'react';
 import { SvgUri } from 'react-native-svg';
+import { SvgFallback } from './components/SvgFallback';
 
 export default () => {
   const [uri, setUri] = React.useState(
@@ -140,6 +142,7 @@ export default () => {
       width="100%"
       height="100%"
       uri={uri}
+      fallback={<SvgFallback />}
     />
   );
 };