How to check return value from the shell directive

This worked fine for me - based on @eriktous' answer with a minor modification of redirecting stdout as well to skip the output from svn info on a valid svn repo.

SVN_INFO := $(shell svn info . 1>&2 2> /dev/null; echo $$?)
ifneq ($(SVN_INFO),0)
    $(error "Not an SVN repo...")
endif

How about using $? to echo the exit status of the last command?

SVN_INFO := $(shell svn info . 2> /dev/null; echo $$?)
ifeq ($(SVN_INFO),1)
    $(error "Not an SVN repo...")
endif

If you want to preserve the original output then you need to do some tricks. If you are lucky enough to have GNU Make 4.2 (released on 2016-05-22) or later at your disposal you can use the .SHELLSTATUS variable as follows.

var := $(shell echo "blabla" ; false)

ifneq ($(.SHELLSTATUS),0)
  $(error shell command failed! output was $(var))
endif

all:
    @echo Never reached but output would have been $(var)

Alternatively you could use a temporary file or play with Make's eval to store the string and/or the exit code into a Make variable. The example below gets this done but I would certainly like to see a better implementation than this embarrassingly complicated version.

ret := $(shell echo "blabla"; false; echo " $$?")
rc := $(lastword $(ret))
# Remove the last word by calculating <word count - 1> and
# using it as the second parameter of wordlist.
string:=$(wordlist 1,$(shell echo $$(($(words $(ret))-1))),$(ret))

ifneq ($(rc),0)
  $(error shell command failed with $(rc)! output was "$(string)")
endif

all:
    @echo Never reached but output would have been \"$(string)\"