fix(packages): Converters package no longer worked after 0.13.0

Due an inadvertently inverted check.
Also if wasn't resolving the source file name so could not work
with relative paths that would have been acceptable for a resource
not needing conversion.

packages/converters/init.lua

@@ -59,11 +59,12 @@ function package.register (_, sourceExt, targetExt, command)
 end
 
 function package.checkConverters (_, source)
+  local resolvedSrc = SILE.resolveFile(source) or SU.error("Couldn't find file " .. source)
   for _, converter in ipairs(SILE.scratch.converters) do
     local extLen = string.len(converter.sourceExt)
-    if ((string.len(source) > extLen) and
-        (string.sub(source, -extLen) == converter.sourceExt)) then
-      return applyConverter(source, converter)
+    if ((string.len(resolvedSrc) > extLen) and
+        (string.sub(resolvedSrc, -extLen) == converter.sourceExt)) then
+      return applyConverter(resolvedSrc, converter)
     end
   end
   return source -- No conversion needed.
@@ -75,17 +76,23 @@ function package:_init ()
     SILE.scratch.converters = {}
   end
   extendCommand("include", function (options, content, original)
-    local result = self:checkConverters(options.src)
-    if not result then
+    local source = SU.required(options, "src", "include (converters)")
+    local result = self:checkConverters(source)
+    if result then
       options["src"] = result
       original(options, content)
+    else
+      SU.error("Conversion failure for include '" .. source ..'"')
     end
   end)
   extendCommand("img", function (options, content, original)
-    local result = self:checkConverters(options.src)
-    if not result then
+    local source = SU.required(options, "src", "img (converters)")
+    local result = self:checkConverters(source)
+    if result then
       options["src"] = result
       original(options, content)
+    else
+      SU.error("Conversion failure for image '" .. source ..'"')
     end
   end)
   self:deprecatedExport("register", self.register)