chore(merge-script): add check for number and a useful failure message

Message appears only if merge was unsuccessful and makes deleting
as quick as copypasting.

Check makes sure that supplied argument is actually an number, which
helps avoid waiting for git to eror out on "no remote branch".

Additionally, renamed `original` upstream to `upstream`, according to
@sudden6's suggestion.

merge-pr.sh

@@ -41,13 +41,17 @@ $2
 fi
 
 
-# FIXME: ↓
-#[[ ${PR} =~ "^[:digit:]+$" ]] || echo "Not a PR number!" && exit 1
+# check if supplied var is a number; if not exit
+if [[ ! "${PR}" =~ ^[[:digit:]]+$ ]]
+then
+    echo "Not a PR number!" && \
+    exit 1
+fi
 
 # list remotes, and if there's no tux3 one, add it
-if  ! git remote | grep original > /dev/null
+if  ! git remote | grep upstream > /dev/null
 then
-    git remote add original git@github.com:tux3/qTox.git
+    git remote add upstream git@github.com:tux3/qTox.git
 fi
 
 # print the message only if the merge was successful
@@ -58,20 +62,30 @@ after_merge_msg() {
     echo "git diff master..merge$PR"
     echo ""
     echo "To push that to master on github:"
-    echo "git checkout master && git merge --ff merge$PR && git push original master"
+    echo "git checkout master && git merge --ff merge$PR && git push upstream master"
     echo ""
     echo "After pushing to master, delete branches:"
     echo ""
     echo "git branch -d {merge,}$PR"
 }
 
+# print the message only if some merge step failed
+after_merge_failure_msg() {
+    echo ""
+    echo "Merge failed."
+    echo ""
+    echo "You may want to remove not merged branches, if they exist:"
+    echo ""
+    echo "git checkout master && git branch -D {merge,}$PR"
+}
+
 
-git fetch original && \
+git fetch upstream && \
 git checkout master && \
-git rebase original/master master && \
-git fetch original pull/$PR/head:$PR && \
+git rebase upstream/master master && \
+git fetch upstream pull/$PR/head:$PR && \
 git checkout master -b merge$PR && \
 git merge --no-ff -S $PR -m "Merge pull request #$PR
 $OPT_MSG
 $(git shortlog master..$PR)" && \
-after_merge_msg
+after_merge_msg || after_merge_failure_msg