unminify: Fix lookup if source map does not exist in disk.
unminify: Fix lookup if source map does not exist in disk.

If the client has an old version of the code which is not present on
the server, don't throw a 500; instead, default to the same `unable to
look up in source map` message is used when the line numbers don't
line up.

zulip/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,andersk/zulip,hackerkid/zulip,rht/zulip,hackerkid/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,punchagan/zulip,kou/zulip,kou/zulip,rht/zulip,rht/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,punchagan/zulip