Better error when `TagHelperOutputExtensions.CopyHtmlAttribute()` called with unknown `attributeName`

[dougbu]

If the provided TagHelperContext does not contain attributeName, tag helper authors get very little information on how to fix the problem. Should improve the error message and help these authors out.

Details: An example of the current error:

System.InvalidOperationException : Sequence contains no matching element
Stack Trace:
  at System.Linq.Enumerable.First[TSource](IEnumerable`1 source, Func`2 predicate)
  at Microsoft.AspNet.Mvc.TagHelpers.TagHelperOutputExtensions.CopyHtmlAttribute(TagHelperOutput tagHelperOutput, String attributeName, TagHelperContext context) in C:\dd\k\Mvc.0\src\Microsoft.AspNet.Mvc.TagHelpers\TagHelperOutputExtensions.cs:line 33
  at Microsoft.AspNet.Mvc.TagHelpers.InputTagHelper.Process(TagHelperContext context, TagHelperOutput output) in C:\dd\k\Mvc.0\src\Microsoft.AspNet.Mvc.TagHelpers\InputTagHelper.cs:line 114
  at Microsoft.AspNet.Razor.Runtime.TagHelpers.TagHelper.<ProcessAsync>d__1.MoveNext()
  --- End of stack trace from previous location where exception was thrown ---
  ...

Could perhaps use FirstOrDefault() and throw a better Exception if that returns null.

[dougbu]

323ec2f