replace "shape error across CALL" with more information.

x.go:7: not enough arguments to CALL
	a int, b int
	int
x.go:10: assignment count mismatch: 3 = 2
x.go:12: too many arguments to RETURN
	[no arguments expected]
	int, int, int

also leave type alone after conversion failure,
for later errors:

bug049.go:6: cannot convert nil constant to string
bug049.go:6: illegal types for operand: EQ
	string
	nil		# this used to be blank

R=ken
OCL=28405
CL=28407