golang: error test execution when "go" binary was not found

This prevents an error when the avocado golang plugin is installed,
but "go" itself is not.

This still allows tests to be listed without "go" because:
 * it is not a requirement for that
 * there's value in listing tests even when your system is (still?)
   not completely setup to run them
 * Avocado hasn't promised symmetry between listing and guaranteeing
   that tests will be able to run
 * There may be situations in which we can *not* predict if tests
   that we can list can be executed
Signed-off-by: NCleber Rosa <crosa@redhat.com>

optional_plugins/golang/avocado_golang/__init__.py

@@ -21,6 +21,7 @@ import glob
 import os
 import re
 
+from avocado.core import exceptions
 from avocado.core import loader
 from avocado.core import output
 from avocado.core import test
@@ -29,7 +30,10 @@ from avocado.utils import path as utils_path
 from avocado.utils import process
 
 
-_GO_BIN = utils_path.find_command('go')
+try:
+    _GO_BIN = utils_path.find_command('go')
+except utils_path.CmdNotFoundError:
+    _GO_BIN = None
 
 
 class GolangTest(test.SimpleTest):
@@ -57,6 +61,9 @@ class GolangTest(test.SimpleTest):
         """
         Create the Golang command and execute it.
         """
+        if _GO_BIN is None:
+            raise exceptions.TestError("go binary not found")
+
         test_name = '%s$' % self.name.name.split(':')[1]
         if self.subtest is not None:
             test_name += '/%s' % self.subtest