Improve TextFormatDecodingError

[thomasvl]

TextFormat shouldn't get a ton of use in end user code, but it is very useful in write test cases, configurations, etc. So having better error for when the input is invalid can be pretty useful. Our current values have no context, but for some of them there are likely some pretty easy additions/wins:

• .unrecognizedEnumValue getting the enum's full name and the string value
• .conflictingOneOf getting the full message name, the already set field, and the field trying to be set.

Also a good number of the errors could actually also use line number (or atleast offset) into the string buffer being parsed to help someone correct their input.

[thomasvl]

main has gotten the infrastructure for a new error model that won't be api breaking, so new errors can have more details. However moving existing errors to that new model was counted as api breaking. So I'm closing this off as we won't actually evolve TextFormatDecodingError , but at some future point with 2.0 work, the existing errors could be given more context like this.