Better error message when :GBrowse remote not found

Resolves: https://github.com/tpope/vim-fugitive/issues/2103

autoload/fugitive.vim

@@ -7524,7 +7524,11 @@ function! fugitive#BrowseCommand(line1, count, range, bang, mods, arg, ...) abor
       endif
     endfor
 
-    throw "fugitive: no GBrowse handler installed for '".raw."'"
+    if !empty(remote_url)
+      return 'echoerr ' . string("fugitive: no GBrowse handler installed for '".remote_url."'")
+    else
+      return 'echoerr ' . string("fugitive: could not find remote named '".remote."'")
+    endif
   catch /^fugitive:/
     return 'echoerr ' . string(v:exception)
   endtry