gnuplot error handling

[ievgrafov]

The gem should handle gnuplot's stderr pipe. Now some errors broke ruby terminal.
min: catch error messages and show them to gem's user

[ievgrafov]

error handling implemented but still may be improved

[mess110]

Could you please provide some examples of what improvements could be done?

[ievgrafov]

How error handling works now:

1. User sends wrong command to gnuplot (e.g. plot.wrong_option('some value') )
2. Gem checks error container for errors and executes command because container is empty
3. Wrong command makes gnuplot to send something to sterr. Another thread in gem is listening stderr for output. Thread takes this output and puts it into container. It takes gnuplot some time (exact value depends on hardware, OS etc) to produce this error so user may execute several command during this time.
4. Finally user runs a command, it checks container for errors and finds something. Gem raises exception in this case.
   As you see exception is raised but not during execution of command that really was wrong. Currently I don't see a way to wait for gnuplot process to execute a command. Possible improvements should do so I think.