Check return codes everywhere

[elfring]

Some checks for return codes are missing.

Examples:
Would you like to add more error handling for return values from "​malloc" like in the function "read_ppm_file" and from "​fwrite" in the function "copy_stream"?

[davea]

I added a few. Not feasible to do them all right now.

[elfring]

I suggest to avoid unchecked function calls.
Would you like to detect every error situation as early as possible?

There are still some ignored error codes from functions like fread.
(By the way: Is it a bug that there is no ​break instruction at the end of "case '3':"? Is the fall-through really intended?)

[Nicolas]

Fall-throughs on switch blocks should always be noticed with a //fallthrough comment in place of the break;.

Also, it's clear it's not feasible to check error codes on every function call. That's why ​handling were invented. BOINC should really start using them...

[Nicolas]

Oops, the link on my previous comment is broken. I meant: That's why​exceptions were invented.

[elfring]

Each return value should be checked to see if an exception object should be thrown. Are there any situations where the reaction "​exit(errno)" or "​abort()" would be more appropriate?

Would you like to reduce the efforts for ​error code checking by an ​exception class hierarchy?