add --json-error-messages (not yet implemented)

Added --json-error-messages option, which includes error messages in the
json output, rather than outputting them to stderr.

The actual rediretion of errors is not implemented yet, this is only
the docs and option plumbing.

This commit was supported by the NSF-funded DataLad project.

Command/MetaData.hs

@@ -23,7 +23,7 @@ import Data.Aeson
 import Control.Concurrent
 
 cmd :: Command
-cmd = withGlobalOptions ([jsonOption] ++ annexedMatchingOptions) $ 
+cmd = withGlobalOptions [jsonOptions, annexedMatchingOptions] $ 
 	command "metadata" SectionMetaData
 		"sets or gets metadata of a file"
 		paramPaths (seek <$$> optParser)