show: Handle read and write errors

show: Handle read and write errors

For showing a message in raw format, rather than silently succeeding
when a read or a write fails (or, probably, looping if a read fails),
try to print an error message and exit with a non-zero status.

This silences one of the buildbot warnings about unused results.  While
my libc lacks the declarations that trigger these warnings, this can
be tested by adding the following to notmuch.h:

__attribute__((warn_unused_result))
size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);