Reworked how lack of Jeweler is handled in Rakefile.
Reworked how lack of Jeweler is handled in Rakefile.

 * Print error message to standard error instead of standard output.
 * Print exception class and message.
 * Split output on multiple lines to fit into terminals better.

thuss/standalone-migrations,valexl/standalone-migrations,thinkerbot/standalone-migrations,beachyapp/standalone-migrations