[SPARK-24929][INFRA] Make merge script don't swallow KeyboardInterrupt

## What changes were proposed in this pull request?

If you want to get out of the loop to assign JIRA's user by command+c (KeyboardInterrupt), I am unable to get out. I faced this problem when the user doesn't have a contributor role and I just wanted to cancel and manually take an action to the JIRA.

**Before:**

```
JIRA is unassigned, choose assignee
[0] todd.chen (Reporter)
Enter number of user, or userid,  to assign to (blank to leave unassigned):Traceback (most recent call last):
  File "./dev/merge_spark_pr.py", line 322, in choose_jira_assignee
    "Enter number of user, or userid,  to assign to (blank to leave unassigned):")
KeyboardInterrupt
Error assigning JIRA, try again (or leave blank and fix manually)
JIRA is unassigned, choose assignee
[0] todd.chen (Reporter)
Enter number of user, or userid,  to assign to (blank to leave unassigned):Traceback (most recent call last):
  File "./dev/merge_spark_pr.py", line 322, in choose_jira_assignee
    "Enter number of user, or userid,  to assign to (blank to leave unassigned):")
KeyboardInterrupt
```

**After:**

```
JIRA is unassigned, choose assignee
[0] Dongjoon Hyun (Reporter)
Enter number of user, or userid to assign to (blank to leave unassigned):Traceback (most recent call last):
  File "./dev/merge_spark_pr.py", line 322, in choose_jira_assignee
    "Enter number of user, or userid to assign to (blank to leave unassigned):")
KeyboardInterrupt
Restoring head pointer to master
git checkout master
Already on 'master'
git branch
```

## How was this patch tested?

I tested this manually (I use my own merging script with few fixes).

Author: hyukjinkwon <[email protected]>

Closes apache#21880 from HyukjinKwon/key-error.

Author: HyukjinKwon

dev/merge_spark_pr.py

@@ -319,7 +319,7 @@ def choose_jira_assignee(issue, asf_jira):
                     annotations.append("Commentor")
                 print("[%d] %s (%s)" % (idx, author.displayName, ",".join(annotations)))
             raw_assignee = input(
-                "Enter number of user, or userid,  to assign to (blank to leave unassigned):")
+                "Enter number of user, or userid, to assign to (blank to leave unassigned):")
             if raw_assignee == "":
                 return None
             else:
@@ -331,6 +331,8 @@ def choose_jira_assignee(issue, asf_jira):
                     assignee = asf_jira.user(raw_assignee)
                 asf_jira.assign_issue(issue.key, assignee.key)
                 return assignee
+        except KeyboardInterrupt:
+            raise
         except:
             traceback.print_exc()
             print("Error assigning JIRA, try again (or leave blank and fix manually)")