Conventions for error reason string

[jclark]

We need to decide on some conventions for the format of strings used for error reasons. This affects

• language, for error reason strings in language-generated panics
• lang library, for errors returned by the lang library (see Define some standard error types for use by lang.* modules #134)
• platform, for standard library

[jclark]

Current idea is to use format {org/module}CamelCase.

[jclark]

Language defined errors will have the format {ballerina/lang.*}CamelCase.