Make LoadError from running tests more obvious

[drbrain]

When you attempt to run tests on a nonexistent file the error can be buried (see #194). This change adds some whitespace and removes the backtrace to make the error more visible.

$ rake test TEST=no_such_file
/usr/local/bin/ruby -w -I"lib:test" -I"/Users/erichodel/Work/git/rake/lib" "/Users/erichodel/Work/git/rake/lib/rake/rake_test_loader.rb" "no_such_file" 

cannot load such file -- /Users/erichodel/Work/git/rake/no_such_file

rake aborted!
Command failed with status (1): [ruby -w -I"lib:test" -I"/Users/erichodel/Work/git/rake/lib" "/Users/erichodel/Work/git/rake/lib/rake/rake_test_loader.rb" "no_such_file" ]

Tasks: TOP => test
(See full trace by running task with --trace)

[coveralls]

Coverage decreased (-0.02%) to 96.553% when pulling 48a9c46 on drbrain/better-load-error into af15762 on master.

[coveralls]

Coverage decreased (-0.01%) to 96.561% when pulling 66ec746 on drbrain/better-load-error into af15762 on master.

[coveralls]

Coverage decreased (-0.01%) to 96.561% when pulling fdc443a on drbrain/better-load-error into af15762 on master.

[coveralls]

Coverage decreased (-0.03%) to 96.541% when pulling 8d3a682 on drbrain/better-load-error into af15762 on master.

[drbrain]

@coveralls Lies: this implementation is covered here

[hsbt]

and I set coverage decrease threshold rate to 1%.

[drbrain]

Someday I will circle back and report this bug to SimpleCov