Fix mishandled URL parsing error in resolution spec

URL parsing "failure" signal needs to be converted into import maps spec "null" signal.

spec.bs

@@ -295,7 +295,11 @@ To <dfn>register an import map</dfn> given a [=string=] or a null |source text|,
 <div algorithm>
   To <dfn lt="parse a URL-like import specifier|parsing a URL-like import specifier">parse a URL-like import specifier</dfn>, given a [=string=] |specifier| and a [=URL=] |baseURL|:
 
-  1. If |specifier| [=/starts with=] "`/`", "`./`", or "`../`", then return the result of [=URL parser|parsing=] |specifier| with |baseURL| as the base URL.
+  1. If |specifier| [=/starts with=] "`/`", "`./`", or "`../`", then:
+    1. Let |url| be the result of [=URL parser|parsing=] |specifier| with |baseURL| as the base URL.
+    1. If |url| is failure, then return null.
+       <p class="example" id="example-bad-urllike-import-specifier">One way this could happen is if |specifier| is "`../foo`" and |baseURL| is a `data:` URL.</p>
+    1. Return |url|.
   1. Let |url| be the result of [=URL parser|parsing=] |specifier| (with no base URL).
   1. If |url| is failure, then return null.
   1. If |url|'s [=url/scheme=] is either a [=fetch scheme=] or "`std`", then return |url|.
@@ -334,7 +338,7 @@ To <dfn>register an import map</dfn> given a [=string=] or a null |source text|,
 
 All call sites of HTML's existing <a spec="html">resolve a module specifier</a> will need to be updated to pass the appropriate [=script=], not just its [=script/base URL=].
 
-They will also need to be updated to account for it now throwing exceptions, instead of returning failure. (Previously they just turned failures into {{TypeError}}s manually, so this is straightforward.)
+They will also need to be updated to account for it now throwing exceptions, instead of returning failure. (Previously most call sites just turned failures into {{TypeError}}s manually, so this is straightforward.)
 
 <div algorithm>
   To <dfn lt="resolve an imports match|resolving an imports match">resolve an imports match</dfn>, given a [=string=] |normalizedSpecifier|, a [=specifier map=] |specifierMap|, and a [=module map=] |moduleMap|: