[FIX] portal: properly handle errors

Before this commit, Portal overrode the _render function for its
component node, which means it bypassed the error handling mechanism
that was implemented in that method.  It could have been fixed by
duplicating the error handling code as well, but a better solution in my
opinion is to simply override the renderFn function.  This is closer to
the actual intent of the portal implementation: wrap the result of the
rendering in a VPortal vnode.

tests/misc/__snapshots__/portal.test.ts.snap

@@ -409,6 +409,45 @@ exports[`Portal portal's parent's env is not polluted 2`] = `
 }"
 `;
 
+exports[`Portal simple catchError with portal 1`] = `
+"function anonymous(bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, component, comment } = bdom;
+  
+  let block1 = createBlock(\`<div><block-child-0/><block-child-1/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let b2,b3;
+    if (ctx['error']) {
+      b2 = text(\`Error\`);
+    } else {
+      b3 = component(\`Boom\`, {}, key + \`__1\`, node, ctx);
+    }
+    return block1([], [b2, b3]);
+  }
+}"
+`;
+
+exports[`Portal simple catchError with portal 2`] = `
+"function anonymous(bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, component, comment } = bdom;
+  
+  let block1 = createBlock(\`<div><span>1</span><block-child-0/></div>\`);
+  let block2 = createBlock(\`<p><block-text-0/></p>\`);
+  
+  function slot1(ctx, node, key = \\"\\") {
+    let txt1 = ctx['a'].b.c;
+    return block2([txt1]);
+  }
+  
+  return function template(ctx, node, key = \\"\\") {
+    let b3 = component(\`Portal\`, {target: '#outside',slots: {'default': {__render: slot1, __ctx: ctx}}}, key + \`__1\`, node, ctx);
+    return block1([], [b3]);
+  }
+}"
+`;
+
 exports[`Portal with target in template (after portal) 1`] = `
 "function anonymous(bdom, helpers
 ) {