When reporting "sorry, *this* packet can't be written to a file of that

When reporting "sorry, *this* packet can't be written to a file of that
type" when writing out a capture file (i.e., writing a
per-packet-encapsulation capture to a file type that supports it but
doesn't support one of the packet's encapsulations), report the packet
number and, when doing this in a merge operation, report the file from
which it came.

When reporting "sorry, that file can't be written to a file of that
type, period", show the file type rather than the input file link-layer
type that causes the problem.  (We could show both.  We could be
*really* ambitious and iterate through all possible file types and show
the ones that will or at least might work....)

file_write_error_message() is documented as handling only UNIX-style
errnos, and libwireshark should be usable without libwiretap, so leave
it up to its callers to handle Wiretap errors such as
WTAP_ERR_SHORT_WRITE.

Clean up indentation.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@39949 f5534014-38df-0310-8fa8-9805f1628bb7