#1 NSError support for AGRegex

[Sherm Pendley]

This patch adds two new methods to AGRegex:

+ (id)regexWithPattern:(NSString *)pat options:(int)opts error:(NSError
**)error;
- (id)initWithPattern:(NSString *)pat options:(int)opts error:(NSError **)
error;

If pattern compilation fails, and error is non-null, an NSError object is
created with the error message returned by pcre_compile(), and
returned in the NSError* pointed to by error.

Also, Controller.m is modified to use this, creating and displaying an
NSAlert with the returned NSError.

[Sherm Pendley]

NSError support for AGRegex