Step 1: understand/review what the current help message says and what kind of support it provides
Step 2: improve it to help people understand what they need to do in order to be able to import a file from their wiki using FileImporter
For step 2: Update the error message as follows:
"Unfortunately, importing files from the source wiki ($1) is not yet possible because there is no configuration for the wiki at $2. For information about setting up a configuration file for the wiki, review FileImporter's [configuration file documentation page | ($3)]."
The parameters should be filled with:
($1) host name of the source wiki (e.g. "en.wikipedia.org") i.e. no change to current behaviour
($2) https://www.mediawiki.org/wiki/Extension:FileImporter/Data i.e. no change to current behaviour
($3) https://meta.wikimedia.org/wiki/WMDE_Technical_Wishes/Move_files_to_Commons/Configuration_file_documentation (new embedded link under "configuration file documentation page")
In addition, we thought it would be useful to update the warning message at the top of https://www.mediawiki.org/wiki/Extension:FileImporter/Data to say the following:
"In order to import files from a wiki, a configuration file must be set up as a subpage of this page and be listed below. Changing any subpage will change the behavior of FileImporter handling specific files. For information on these configuration file subpages, please visit the [documentation page | ($3)]. Please also note that when adding a subpage, the title should follow domain name minus ".org", for example, the one for Dutch Wikipedia should use "nl.wikipedia"."