Move dependency check so we can send our metadata

I missed this detail about the module.run method when adding the check.
Defining the metadata or where you put it doesn't matter so much as if
you're sending it over JSON-RPC.

modules/exploits/windows/smb/ms17_010_eternalblue_win8.py

@@ -8,9 +8,9 @@ from base64 import b64decode
 try:
     from impacket import smb, ntlm
 except ImportError:
-    no_impacket = True
+    dependencies_missing = True
 else:
-    no_impacket = False
+    dependencies_missing = False
 
 from metasploit import module
 
@@ -647,6 +647,10 @@ def _exploit(target, port, feaList, shellcode, numGroomConn, username, password)
 
 
 def exploit(args):
+    if dependencies_missing:
+        module.log('Module dependencies (impacket) missing, cannot continue', 'error')
+        return
+
     # XXX: Normalize strings to ints and unset options to empty strings
     rport = int(args['RPORT'])
     numGroomConn = int(args['GroomAllocations'])
@@ -671,8 +675,4 @@ def exploit(args):
 
 
 if __name__ == '__main__':
-    if no_impacket:
-        module.log('Please install Impacket before continuing', 'error')
-        sys.exit(1)
-
     module.run(metadata, exploit)