kunit: tool: fix display of make errors

CalledProcessError stores the output of the failed process as `bytes`,
not a `str`.

So when we log it on build error, the make output is all crammed into
one line with "\n" instead of actually printing new lines.

After this change, we get readable output with new lines, e.g.
>   CC      lib/kunit/kunit-example-test.o
> In file included from ../lib/kunit/test.c:9:
> ../include/kunit/test.h:22:1: error: unknown type name ‘invalid_type_that_causes_compile’
>    22 | invalid_type_that_causes_compile errors;
>       | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> make[3]: *** [../scripts/Makefile.build:283: lib/kunit/test.o] Error 1

Secondly, trying to concat exceptions to strings will fail with
> TypeError: can only concatenate str (not "OSError") to str
so fix this with an explicit cast to str.
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/kunit/kunit_kernel.py

@@ -36,9 +36,9 @@ class LinuxSourceTreeOperations(object):
 		try:
 			subprocess.check_output(['make', 'mrproper'], stderr=subprocess.STDOUT)
 		except OSError as e:
-			raise ConfigError('Could not call make command: ' + e)
+			raise ConfigError('Could not call make command: ' + str(e))
 		except subprocess.CalledProcessError as e:
-			raise ConfigError(e.output)
+			raise ConfigError(e.output.decode())
 
 	def make_olddefconfig(self, build_dir, make_options):
 		command = ['make', 'ARCH=um', 'olddefconfig']
@@ -49,9 +49,9 @@ class LinuxSourceTreeOperations(object):
 		try:
 			subprocess.check_output(command, stderr=subprocess.STDOUT)
 		except OSError as e:
-			raise ConfigError('Could not call make command: ' + e)
+			raise ConfigError('Could not call make command: ' + str(e))
 		except subprocess.CalledProcessError as e:
-			raise ConfigError(e.output)
+			raise ConfigError(e.output.decode())
 
 	def make_allyesconfig(self, build_dir, make_options):
 		kunit_parser.print_with_timestamp(
@@ -84,9 +84,9 @@ class LinuxSourceTreeOperations(object):
 		try:
 			subprocess.check_output(command, stderr=subprocess.STDOUT)
 		except OSError as e:
-			raise BuildError('Could not call execute make: ' + e)
+			raise BuildError('Could not call execute make: ' + str(e))
 		except subprocess.CalledProcessError as e:
-			raise BuildError(e.output)
+			raise BuildError(e.output.decode())
 
 	def linux_bin(self, params, timeout, build_dir, outfile):
 		"""Runs the Linux UML binary. Must be named 'linux'."""