download: catch exceptions checking for size

Happens on at least one windows box

src/common/download.cpp

@@ -107,6 +107,8 @@ namespace tools
             MINFO("Content-Length: " << length);
             content_length = length;
             boost::filesystem::path path(control->path);
+            try
+            {
               boost::filesystem::space_info si = boost::filesystem::space(path);
               if (si.available < (size_t)content_length)
               {
@@ -115,6 +117,8 @@ namespace tools
                 return false;
               }
             }
+            catch (const std::exception &e) { MWARNING("Failed to check for free space: " << e.what()); }
+          }
           if (offset > 0)
           {
             // we requested a range, so check if we're getting it, otherwise truncate