Added formatted error functions and tests

Added new functions for formatted error creation: Newf and NewSkipCallerf in errors.go. These functions allow error messages with formatted strings. Cleaned up the message building functionality; separated message building from cleaning and formatting. Corresponding tests were also added in errors_test.go.

Author: Gabriel Cataldo

README.md

@@ -3,7 +3,7 @@ Go Errors Detail
 <!--suppress ALL -->
 <img align="right" src="gopher-debug.png" alt="">
 
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