send_common: Improve error diagnostics of generic_com_on().

Currently the function returns an error code but does not log a message
on errors. The callers, http_send_init() and dccp_send_init(), print
the strerror text with no indication that it is was generic_com_on()
which caused the error.

This commit moves the log messages from the callers to generic_com_on()
and changes the return type of generic_com_on() to void because both
callers now ignore the return value. The new log messages include a
text which tells the user what went wrong.