implement for all downloaders and check file existence before removing

ez_setup.py

@@ -162,7 +162,12 @@ def download_file_powershell(url, target):
         '-Command',
         "(new-object System.Net.WebClient).DownloadFile(%(url)r, %(target)r)" % vars(),
     ]
-    subprocess.check_call(cmd)
+    try:
+        subprocess.check_call(cmd)
+    except subprocess.CalledProcessError:
+        if os.access(target, os.F_OK):
+            os.unlink(target)
+        raise
 
 def has_powershell():
     if platform.system() != 'Windows':
@@ -182,7 +187,12 @@ download_file_powershell.viable = has_powershell
 
 def download_file_curl(url, target):
     cmd = ['curl', url, '--silent', '--output', target]
-    subprocess.check_call(cmd)
+    try:
+        subprocess.check_call(cmd)
+    except subprocess.CalledProcessError:
+        if os.access(target, os.F_OK):
+            os.unlink(target)
+        raise
 
 def has_curl():
     cmd = ['curl', '--version']
@@ -203,7 +213,8 @@ def download_file_wget(url, target):
     try:
         subprocess.check_call(cmd)
     except subprocess.CalledProcessError:
-        os.unlink(target)
+        if os.access(target, os.F_OK):
+            os.unlink(target)
         raise
 
 def has_wget():