coverity: detect and report when the token or project is incorrect

When trying to obtain the MD5 of the Coverity Scan Tool (in order to
decide whether a cached version can be used or a new version has to be
downloaded), it is possible to get a 401 (Authorization required) due to
either an incorrect token, or even more likely due to an incorrect
Coverity project name.

Seeing an authorization failure that is caused by an incorrect project
name was somewhat surprising to me when developing the Coverity
workflow, as I found such a failure suggestive of an incorrect token
instead.

So let's provide a helpful error message about that specifically when
encountering authentication issues.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

.github/workflows/coverity.yml

@@ -80,7 +80,18 @@ jobs:
                    --fail \
                    --form token='${{ secrets.COVERITY_SCAN_TOKEN }}' \
                    --form project="$COVERITY_PROJECT" \
-                   --form md5=1) &&
+                   --form md5=1)
+          case $? in
+          0) ;; # okay
+          *22*) # 40x, i.e. access denied
+            echo "::error::incorrect token or project?" >&2
+            exit 1
+            ;;
+          *) # other error
+            echo "::error::Failed to retrieve MD5" >&2
+            exit 1
+            ;;
+          esac
           echo "hash=$MD5" >>$GITHUB_OUTPUT
 
       # Try to cache the tool to avoid downloading 1GB+ on every run.