Return full error message from xmllint (#586)

Update call_command function to include stderr output in CCPPError message.

Right now, when xmllint-ing is done via xml_tools.py (validate_xml_file),
the output of the linter is not returned. This PR adds the stderr output
(if any) to the returned error message.

PR also decodes error messages for cleaner output.

User interface changes?: No

Testing:
test removed: N/A
unit tests: Added new doctest to test error message
system tests: N/A
manual testing: Ran with/parsed new error messages within CAM-SIMA

scripts/parse_tools/xml_tools.py

@@ -51,6 +51,21 @@ def call_command(commands, logger, silent=False):
     False
     >>> call_command(['ls'], _LOGGER)
     True
+    >>> try:
+    ...    call_command(['ls','--invalid-option'], _LOGGER)
+    ... except CCPPError as e:
+    ...    print(str(e))
+    Execution of 'ls --invalid-option' failed with code: 2
+    Error output: ls: unrecognized option '--invalid-option'
+    Try 'ls --help' for more information.
+    >>> try:
+    ...    os.chdir(os.path.dirname(__file__))
+    ...    call_command(['ls', os.path.basename(__file__), 'foo.bar.baz'], _LOGGER)
+    ... except CCPPError as e:
+    ...    print(str(e))
+    Execution of 'ls xml_tools.py foo.bar.baz' failed with code: 2
+    xml_tools.py
+    Error output: ls: cannot access 'foo.bar.baz': No such file or directory
     """
     result = False
     outstr = ''
@@ -66,9 +81,17 @@ def call_command(commands, logger, silent=False):
             result = False
         else:
             cmd = ' '.join(commands)
-            emsg = "Execution of '{}' failed with code:\n"
-            outstr = emsg.format(cmd, err.returncode)
-            outstr += "{}".format(err.output)
+            outstr = f"Execution of '{cmd}' failed with code: {err.returncode}\n"
+            outstr += f"{err.output.decode('utf-8', errors='replace').strip()}"
+            if hasattr(err, 'stderr') and err.stderr:
+                stderr_str = err.stderr.decode('utf-8', errors='replace').strip()
+                if stderr_str:
+                    if err.output:
+                        outstr += os.linesep
+                    # end if
+                    outstr += f"Error output: {stderr_str}"
+                # end if
+            # end if
             raise CCPPError(outstr) from err
         # end if
     # end of try