Avoid throwing validation error on invalid URIs

The recent update of jsonschema inadvertently caused extra validation of
`uri-reference` formatted properties. This change ensures that these
errors are converted to warnings.

Note that we cannot revert the change to jsonschema since the old
version does not handle `uniqueItems` correctly.

src/testdata/with-invalid-uri.sarif

@@ -0,0 +1,42 @@
+{
+  "$schema": "https://raw.githubusercontent.com/oasis-tcs/sarif-spec/master/Schemata/sarif-schema-2.1.0.json",
+  "version": "2.1.0",
+  "runs": [
+    {
+      "tool": {
+        "driver": {
+          "name": "LGTM.com",
+          "organization": "Semmle",
+          "version": "1.24.0-SNAPSHOT",
+          "rules": []
+        }
+      },
+      "results" : [ {
+        "ruleId" : "js/unused-local-variable",
+        "ruleIndex" : 0,
+        "message" : {
+          "text" : "Unused variable foo."
+        },
+        "locations" : [ {
+          "physicalLocation" : {
+            "artifactLocation" : {
+              "uri" : "not a valid URI",
+              "uriBaseId" : "%SRCROOT%",
+              "index" : 0
+            },
+            "region" : {
+              "startLine" : 2,
+              "startColumn" : 7,
+              "endColumn" : 10
+            }
+          }
+        } ]
+      } ],
+      "columnKind": "utf16CodeUnits",
+      "properties": {
+        "semmle.formatSpecifier": "2.1.0",
+        "semmle.sourceLanguage": "java"
+      }
+    }
+  ]
+}