#11 Check return codes everywhere

[Markus Elfring]

Some checks for return codes are missing.

Examples:
Would you like to add more error handling for return values from "fputs" like in the function "putstr" and from "fwrite" in the function "putchars"?
http://tinyscheme.cvs.sourceforge.net/tinyscheme/tinyscheme/scheme.c?revision=1.10&view=markup