Fix leaks after unsuccessful assembly load (another try) (#68502)

* simpler fix

* Account for MONO and WASM differences when probing files

* comment

* Use TARGET_BROWSER instead

* '\\' should be normalized already by the  `GetFullPath`

src/libraries/System.Private.CoreLib/src/Resources/Strings.resx

@@ -2153,6 +2153,9 @@
   <data name="FileNotFound_ResolveAssembly" xml:space="preserve">
     <value>Could not resolve assembly '{0}'.</value>
   </data>
+  <data name="FileNotFound_LoadFile" xml:space="preserve">
+    <value>Could not load file or assembly '{0}'. The system cannot find the file specified.</value>
+  </data>
   <data name="Format_AttributeUsage" xml:space="preserve">
     <value>Duplicate AttributeUsageAttribute found on attribute type {0}.</value>
   </data>

src/libraries/System.Private.CoreLib/src/System/Reflection/Assembly.cs

@@ -263,6 +263,12 @@ public static Assembly LoadFile(string path)
                 if (s_loadfile.TryGetValue(normalizedPath, out result))
                     return result;
 
+                // we cannot check for file presence on BROWSER. The files could be embedded and not physically present.
+#if !TARGET_BROWSER
+                if (!File.Exists(normalizedPath))
+                    throw new FileNotFoundException(SR.Format(SR.FileNotFound_LoadFile, normalizedPath), normalizedPath);
+#endif // !TARGET_BROWSER
+
                 AssemblyLoadContext alc = new IndividualAssemblyLoadContext($"Assembly.LoadFile({normalizedPath})");
                 result = alc.LoadFromAssemblyPath(normalizedPath);
                 s_loadfile.Add(normalizedPath, result);